drm/amdgpu: Fix the comment in amdgpu_gmc.h

More accurate words are used to address a
code review feedback

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>