drm/amdgpu: make a correction on comment

Use a generic comment for AMDGPU_VM_RESERVED_VRAM size.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>