drm/i915: Move initial plane vblank wait into display code

drm/i915: Move initial plane vblank wait into display code

The initial plane vblank wait operates on display registers,
so it really belongs in the display code proper. Move it there.

We can use intel_parent_irq_enabled() to determine if we can
rely on interrupts or not.

On average we should end up waiting half a frame here, so the
polling interval can be fairly long. 1 ms (which actually
makes poll_timeout_us() use ~250-1000 usec) seems good enough
to me.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260410150449.9699-6-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>