Revert "drm/amdgpu: Refactor flip into prepare submit and submit. (v3)"

This reverts commit cb341a31.

The purpose of the refactor was for amdgpu_crtc_prepare/submit_flip to
be used by the DC code, but that's no longer the case.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>