drm/amdgpu: Fix check for RAS support

Use positive logic to check for RAS
support. Rename the function to actually indicate
what it is testing for. Essentially, make the
function a predicate with the correct name.

Cc: Stanley Yang <Stanley.Yang@amd.com>
Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>