Use correct FB handle in radeon_do_pageflip

We were always using the handle of the client provided FB, which
prevented RandR transforms from working, and could result in a black
screen.

Bugzilla: https://bugs.freedesktop.org/107519


Fixes: 740f0850 "Store FB for each CRTC in drmmode_flipdata_rec"
(Ported from amdgpu commit f6cd72e64e85896b6d155bee0930e59771dcb701)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>