drm/i915/dsb: Inline dsb_vblank_delay() into intel_dsb_wait_for_delayed_vblank()

drm/i915/dsb: Inline dsb_vblank_delay() into intel_dsb_wait_for_delayed_vblank()

Drop the now single-use dsb_vblank_delay() helper and inline its logic
directly into intel_dsb_wait_for_delayed_vblank().

This will help to keep all VRR related wait stuff in one place.

v2: Use intel_scanlines_to_usecs() in intel_dsb_wait_usec(). (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250925022352.3129859-1-ankit.k.nautiyal@intel.com