riva_xaa: don't build RivaSetRopPattern when XAA is not supported

Fixes gcc warnings:

riva_xaa.c: In function ‘RivaSetRopPattern’:
riva_xaa.c:94:33: warning: implicit declaration of function
  ‘XAAGetPatternROP’; did you mean ‘RivaSetPattern’?
  [-Wimplicit-function-declaration]
         pRiva->riva.Rop->Rop3 = XAAGetPatternROP(rop);
                                 ^~~~~~~~~~~~~~~~
                                 RivaSetPattern
At top level:
riva_xaa.c:89:1: warning: ‘RivaSetRopPattern’ defined but not used
  [-Wunused-function]
 RivaSetRopPattern(RivaPtr pRiva, int rop)
 ^~~~~~~~~~~~~~~~~

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>