drm/i915: Make DSB lower level

drm/i915: Make DSB lower level

We could have many different uses for the DSB(s) during a
single commit, so the current approach of passing the whole
crtc_state to the DSB functions is far too high level. Lower
the abstraction a little bit so each DSB user can decide where
to stick the command buffer/etc.

v2: Document the intel_dsb_prepare() return value (Ankit)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123152638.20622-10-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>