drm/i915/dsb: Introduce intel_dsb_wait_vblanks()

Add a function to emit a DSB wait for vblank instruction. This
just waits until the specified number of vblanks.

Note that this triggers on the transcoder's undelayed vblank,
as opposed to the pipe's delayed vblank.

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/20240930170415.23841-8-ville.syrjala@linux.intel.com