drm/i915/dsb: Fix dewake scanline

drm/i915/dsb: Fix dewake scanline

Currently we calculate the DEwake scanline based on
the delayed vblank start, while in reality it should be computed
based on the undelayed vblank start (as that is where the DSB
actually starts). Currently it doesn't really matter as we
don't have any vblank delay configured, but that may change
in the future so let's be accurate in what we do.

We can also remove the max() as intel_crtc_scanline_to_hw()
can deal with negative numbers, which there really shouldn't
be anyway.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624191032.27333-7-ville.syrjala@linux.intel.com
Reviewed-by: Animesh Manna <animesh.manna@intel.com>