drm/i915/dsb: Garbage collect the MMIO DEwake stuff

Since the introduction of DSB chaining we no longer need the
DEwake tricks in intel_dsb_commit().

I also need to relocate the DSB_PMCTRL* writes out of
intel_dsb_finish() (due to the flip queue DMC vs. DSB register
corruption issues), and it'll be a bit more straightforward if
I don't have to worry about the non-chained DSB path anymore.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250612145018.8735-5-ville.syrjala@linux.intel.com