drm/i915: Check vblank delay validity

drm/i915: Check vblank delay validity

Make sure we have enough vblank for the computed vblank delay.
Supposedly we'd reject things anyway later if this gets violated,
but it seems nicer to do some basic sanity checks early just
so we can be sure the basic relationship vblank_end > vblank_start
always holds.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241210211007.5976-3-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>