drm/i915/display: remove intel_wait_for_vblank()

There are only three call sites remaining for
intel_wait_for_vblank(). Remove the function, and open code it to avoid
new users from showing up.

v2:
- Use intel_crtc_wait_for_next_vblank() (Ville)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/aabcda1208072a732d7796e6dacce37dca9bb843.1638366969.git.jani.nikula@intel.com