Why do you think his power comes from a higher being? I've always presumed there will be no literal gods within ASOIAF

The power is derived from those who benefit from it most - the Keyholders of the Iron Bank (Team evil Red). Now just like there are singers in the caves of the Children of the Forest doing their singsong thing in order to preserve the natural magic of the North and Westeros, there are also something similar the Iron Bank (well, the Iron Bank is more of a servant of evil that THE EVIL itself) has ties to or is one component of what lies in Stygai - Stygai is the home of Death - and R'hllor if there is such a beast - I think Drogon is R'hllor in flesh form, much like Baeleron the Black Dread was (spelling?) - there are singers of death in the fiery pits of Stygai - this is in-part why all men must die - to contribute to the powers of evil and become the magical singers of evil -- the only problem is this- when everyone dies there is no one but the dead to rule - haha - kinda self-defeats their own purposes - unless they want to bring about a world of complete death - if you catch my drift

in fact I think GRRM has said this

GRRM walks a narrow tightrope trying not to be too preachy and expose his either agnostic leaning/atheist and almost natural pagan beliefs as this would influence his marketability - he is surely using many Marxist themes in regards to use of formalized religion as a control mechanism (opiate of the masses). I'm not going into this more as it's a beaten horse by now.

receives his prophecies/visions by drinking the wine of the warlocks

Ah yes! Good reminder. (this is why I think Cesrei is an effective survivor (and secretly a Red Priestess)- but she has forgotten her true mission -- she is really a high powerful Red Priestess (by blood) but again, has forgotten this power - she has the wino part down pat (as does Tyrion), but she doesn't know what he is doing- similar to Bran and Dany (but Bran is learning) and Dany now has a few guides (Red Priest/Priestresses in show) - whatever their motivations are to use her dragons and power. Go back and think about when Cersei killed Melara Hetherspoon (REF: movie Heathers, btw) - Cersei was using a blood sacrifice of sorts in order to try to reverse the curse (in her own mind) of Maggy the Frog (maegi) using what she thought to be a valid means inherent to her self, but perhaps not fully known - Cersei, obviously, is true evil oriented