drm/amdgpu: move taking mmap_sem into get_user_pages v2

This didn't helped as intended, just simplify the code.

v2: unlock mmap_sem in the error path as well

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