These should be functionally equivalent to the older per/post modeset
functions, except that they block out drm_vblank_get right away.
There's only the clock adjusting code (outside of pageflips) in
readone which uses drm_vblank_get. But that code doesn't synchronize
against concurrent modesets and instead handles any such races by
waiting for the right vblank to arrive with a short timetout.

The longer-term plan here is to switch all kms drivers to
drm_vblank_on/off so that common code like pending event cleanup can
be done there, while drm_vblank_pre/post_modeset will be purely
drm internal for the old UMS ioctl.

Note that with this patch Michel uncovered a bug in the dri3
implementation of the DDX (it does vblank waits when the pipe is off),
which had to be fixed first.

Cc: Michel Dänzer <michel.daenzer@amd.com>
Acked-and-tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>