Now that we unconditionally dtrt when disabling/enabling crtcs we
don't need any hacks any longer to keep the vblank logic sane when
all the registers go poof. So let's rip it all out.

This essentially undoes

commit 9dbd8feb
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Tue Jul 23 10:48:11 2013 -0300

    drm/i915: update last_vblank when disabling the power well

Apparently igt/kms_flip is already powerful enough to exercise this
properly, yay! See the reference regression report for details.

v2: Update testcase name

References: https://bugs.freedesktop.org/show_bug.cgi?id=66808
Testcase: igt/kms_flip/vblank-vs-*-rpm
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>