drm/radeon: Move fb update from radeon_flip_work_func to radeon_crtc_page_flip

Fixes WARN()s from the DRM core since the page flip rework.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=77521


Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>