The vblank/scanline code is fairly isolated in i915_irq.c. Split it out
to new intel_vblank.[ch].

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3e3e9016f5135dccae5110c291ba048567622e7a.1673873708.git.jani.nikula@intel.com