drm/i915: Convert pxvid to extvid lookup table to a function

The conversion table can be replaced with simple enough function.

   text	   data	    bss	    dec	    hex	filename
 839688	  10987	     24	 850699	  cfb0b	drivers/gpu/drm/i915/i915.ko
 839224	  10987	     24	 850235	  cf93b	drivers/gpu/drm/i915/i915.ko

Result is 494 saved bytes (.05525%).

v2: - no run on sentences from subject (Chris, Jani)
    - be verbose about the savings (Chris, Daniel)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>