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

Make pflip atomic friendly. Split the fuinction into
whatever can fail part and the actual flip submit part.
Call the pre-submit function before atomic states
are swapped so in case of error we can fail the
IOCTL.

v2:
Update due to  target_vblank code change.
Fix identetation.
Change return type for amdgpu_crtc_submit_flip to void
v3: agd: fix formatting

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