drm/i915/dsb: add intel_dsb_gosub_finish()

drm/i915/dsb: add intel_dsb_gosub_finish()

A DSB buffer which will be used for GOSUB execution does not need
the DEWAKE mechanism but still need to be 64 bit aligned. Add helper
to finish preparation of a dsb buffer to be executed with GOSUB
instruction.

v2: Add a cacheline of noops at the end of GOSUB buffer (Ville)

Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Link: https://lore.kernel.org/r/20250523062041.166468-6-chaitanya.kumar.borah@intel.com