drm/amdgpu: Fix tracking of invalid userptrs

Restore the code that resets mem->invalid. Othewise so mapping
userptrs after they got an MMU notifiers would always be skipped.

This also avoids unnecessarily calling get_user_pages on BOs that
have not been invalidated since the last try.

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