drm/i915/dsb: Allow intel_dsb_chain() to use DSB_WAIT_FOR_VBLANK

drm/i915/dsb: Allow intel_dsb_chain() to use DSB_WAIT_FOR_VBLANK

Allow intel_dsb_chain() to start the chained DSB
at start of the undelaye vblank. This is slightly
more involved than simply setting the bit as we
must use the DEwake mechanism to eliminate pkgC
latency.

And DSB_ENABLE_DEWAKE itself is problematic in that
it allows us to configure just a single scanline,
and if the current scanline is already past that
DSB_ENABLE_DEWAKE won't do anything, rendering the
whole thing moot.

The current workaround involves checking the pipe's current
scanline with the CPU, and if it looks like we're about to
miss the configured DEwake scanline we set DSB_FORCE_DEWAKE
to immediately assert DEwake. This is somewhat racy since the
hardware is making progress all the while we're checking it on
the CPU.

We can make things less racy by chaining two DSBs and handling
the DSB_FORCE_DEWAKE stuff entirely without CPU involvement:
1. CPU starts the first DSB immediately
2. First DSB configures the second DSB, including its dewake_scanline
3. First DSB starts the second w/ DSB_WAIT_FOR_VBLANK
4. First DSB asserts DSB_FORCE_DEWAKE
5. First DSB waits until we're outside the dewake_scanline-vblank_start
   window
6. First DSB deasserts DSB_FORCE_DEWAKE

That will guarantee that the we are fully awake when the second
DSB starts to actually execute.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624191032.27333-12-ville.syrjala@linux.intel.com
Reviewed-by: Animesh Manna <animesh.manna@intel.com>