drm/i915/dsb: Improve the indexed reg write checks

drm/i915/dsb: Improve the indexed reg write checks

Currently intel_dsb_indexed_reg_write() just assumes the previous
instructions is also an indexed register write, and thus only
checks the register offset. Make the check more robust by
actually checking the instruction opcode as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216003810.13338-8-ville.syrjala@linux.intel.com
Reviewed-by: Animesh Manna <animesh.manna@intel.com>