sna: Switch from ifdef PICT_ to if XORG_VERSION_CURRENT

The PICT_ are enums and so never report true to ifdef PICT_a2r10g10b10
and instead we need to check the xserver version they were introduced.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>