GART and VRAM size limits need to be a power of two.
Fix values greater than 1GB and simplify those checks a bit.

v2: also fix radeon_vram_limit usage, and simplify test even more.
v3: agd5f: fix spelling as noticed by Klaus Schnass
Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>