amdgpu: fix GEM obj leak in amdgpu_display_user_framebuffer_create

This error code-path is missing a drm_gem_object_put call. Other
error code-paths are fine.

Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: 1769152a

 ("drm/amdgpu: Fail fb creation from imported dma-bufs. (v2)")
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org