drm/i915/dsb: Avoid reads of the DSB buffer for indexed register writes

drm/i915/dsb: Avoid reads of the DSB buffer for indexed register writes

Reading from the DSB command buffer might be somewhat expensive on
discrete GPUs because the buffer resides in GPU local memory. Avoid
such reads in the indexed register write handling by tracking the
previous instruction in intel_dsb.

TODO: actually measure this

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/20240930170415.23841-2-ville.syrjala@linux.intel.com