drm/amdgpu: Remove unused flip_flags from amdgpu_crtc

No more need since Andrey's change to use drm_crtc's version

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>