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’?
         pRiva->riva.Rop->Rop3 = XAAGetPatternROP(rop);
At top level:
riva_xaa.c:89:1: warning: ‘RivaSetRopPattern’ defined but not used
 RivaSetRopPattern(RivaPtr pRiva, int rop)

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