Use radeon_finish in drmmode_crtc_scanout_update

radeon_glamor_finish only works if we're using glamor, otherwise it'll
crash.

Fixes: ce7db510 "Cancel pending scanout update in drmmode_crtc_scanout_update"
Bug: https://bugs.debian.org/924540


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>