drm/i915/dsb: Convert dewake_scanline to a hw scanline number earlier

Currently we switch from out software idea of a scanline
to the hw's idea of a scanline during the commit phase in
_intel_dsb_commit(). While that is slightly easier due to
fastsets fiddling with the timings, we'll also need to
generate proper hw scanline numbers already when emitting
DSB scanline wait instructions. So this approach won't
do in the future. Switch to hw scanline numbers earlier.

Also intel_dsb_dewake_scanline() itself already makes
some assumptions about VRR that don't take into account
VRR toggling during fastsets, so technically delaying
the sw->hw conversion doesn't even help us.

The other reason for delaying the conversion was that we
are using intel_get_crtc_scanline() during intel_dsb_commit()
which gives us the current sw scanline. But this is pretty
low level stuff anyway so just using raw PIPEDSL reads seems
fine here, and that of course gives us the hw scanline
directly, reducing the need to do so many conversions.

v2: Return the non-hw scanline from intel_dsb_dewake_scanline()

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624191032.27333-5-ville.syrjala@linux.intel.com