drm/i915: Simplify scanline_offset handling for gen2

drm/i915: Simplify scanline_offset handling for gen2

Currently intel_crtc_scanline_offset() is careful to always
return a positive offset. That is not actually necessary
as long as we take care of negative values when applying the
offset in __intel_get_crtc_scanline().

This simplifies intel_crtc_scanline_offset(), and makes
the scanline_offfset arithmetic more symmetric between
the forward (__intel_get_crtc_scanline()) and reverse
(intel_crtc_scanline_to_hw()) directions.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240528185647.7765-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>