Revert "drm/radeon: remove drm_vblank_get|put from pflip handling"

This reverts commit 75f36d86.

drm_vblank_get() is necessary to ensure the DRM vblank counter value is
up to date in drm_send_vblank_event().

Seems to fix weston hangs waiting for page flips to complete.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>