drm/radeon: fix and simplify pot argument checks v3
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>