drm/i915: Extract intel_crtc_vblank_delay()

drm/i915: Extract intel_crtc_vblank_delay()

Pull the vblank delay computation into a separate function.
We'll need more logic here soon and we don't want to pollute
intel_crtc_compute_config() with low level details.

We'll use HAS_DSB() to determine if any delay might be required
or not because delayed vblank only really exists for the
purposes of the DSB. It also doesn't event exists on any pre-tgl
platforms, which also don't have DSB. I was midly tempted
to check for the enable_dsb modparam here actually, but as
that can be changed dynamically via debugfs we'd need to either
reconfigure it on the fly or force a modeset. Neither will happen
currently, so we'll just assume DSB may be used of the platform
supports it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241210211007.5976-2-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>