drm/i915: Extract intel_crtc_prepare_vblank_event()

Extract the code for staging the vblank event for the
flip done interrupt handler. We'll reuse this for DSB
stuff later.

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930170415.23841-11-ville.syrjala@linux.intel.com