drm/i915: Extract intel_crtc_scanlines_since_frame_timestamp()

Extract intel_crtc_scanlines_since_frame_timestamp() from
__intel_get_crtc_scanline_from_timestamp(). We'll reuse this
for VRR vblank timestamps.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122232647.22688-5-manasi.d.navare@intel.com