drm/amdgpu: Rename the flags to eliminate ambiguity v2

The flags vf_reg_access_* may cause confusion,
rename the flags to make it more clear.

Signed-off-by: Peng Ju Zhou <PengJu.Zhou@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>