drm/i915: Calculate vblank delay more accurately

drm/i915: Calculate vblank delay more accurately

Calculate the vblank delay in the vblank evasion code correctly
for interlaced modes.

The current code assumes that we won't be using an interlaced mode.
That assumption is actually valid since we've defeatured interlaced
scanout in commit f71c9b7bc35f ("drm/i915/display: Prune Interlace
modes for Display >=12") for DSB capable platforms. However the
feature is still present in the hardware, and if we ever find the
need to re-enable it seems better to calculate the vblank delay
correctly.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624191032.27333-2-ville.syrjala@linux.intel.com
Reviewed-by: Animesh Manna <animesh.manna@intel.com>