drm/amdgpu: Switch error code when bo pin fails. (v3)

Switching to -ENOMEM resolves a hang in case the driver
uses this function in atomic code path.

v2: Update commit message.
v3:
Remove forcing -EINVAL in case of amdgpu_bo_pin fail
since there is no apparent reason for not just passing
through the the return value.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>