drm/i915: Wait for vblank after enabling the primary plane on BDW

BDW signals the flip done interrupt immediately after the DSPSURF write
when the plane is disabled. This is true even if we've already armed
DSPCNTR to enable the plane at the next vblank. This causes major
problems for our page flip code which relies on the flip done interrupts
happening at vblank time.

So what happens is that we enable the plane, and immediately allow
userspace to submit a page flip. If the plane is still in the process
of being enabled when the page flip is issued, the flip done gets
signalled immediately. Our DSPSURFLIVE check catches this to prevent
premature flip completion, but it also means that we don't get a flip
done interrupt when the plane actually gets enabled, and so the page
flip is never completed.

Work around this by re-introducing blocking vblank waits on BDW
whenever we enable the primary plane.

I removed some of the vblank waits here:
 commit 6304cd91
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Fri Apr 25 13:30:12 2014 +0300

    drm/i915: Drop the excessive vblank waits from modeset codepaths

To avoid these blocking vblank waits we should start using the vblank
interrupt instead of the flip done interrupt to complete page flips.
But that's material for another patch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79354


Tested-by: Guo Jinxian <jinxianx.guo@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>