drm/i915/dsb: Generate the DSB buffer in commit_tail()

Once we start using DSB for plane updates we'll need to defer
generating the DSB buffer until the clear color has been
read out. So we need to move at some of the DSB stuff into
commit_tail(). That is perhaps a better place for it anyway
as the ioctl thread can move on immediately without spending
time building the DSB commands.

We always have the MMIO fallback (in case the DSB buffer
allocation fails), so there's no real reason to keep any
of this in the synchronous part of the ioctl.

Because the DSB LUT programming doesn't depend on the plane
clear color we can still do that part before waiting for
fences/etc. which should help paralleize things a bit more.
The DSB plane programming will need to happen after those
however as that depends on the clear color.

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/20240930170415.23841-4-ville.syrjala@linux.intel.com