drm/amdgpu: check base size instead of mem.num_pages

Drop some ussage of mem in the code.

Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210413135248.1266-2-christian.koenig@amd.com


Reviewed-by: Matthew Auld <matthew.auld@intel.com>