xa: fix segfault

Fixes:

  Program received signal SIGSEGV, Segmentation fault.
  bind_samplers (comp=0x21b054, comp=0x21b054, ctx=0x211430)
      at ../../../../../src/gallium/state_trackers/xa/xa_composite.c:445
  445						mask_pic->srf->tex->format);
  (gdb) bt
  #0  bind_samplers (comp=0x21b054, comp=0x21b054, ctx=0x211430)
      at ../../../../../src/gallium/state_trackers/xa/xa_composite.c:445
  #1  xa_composite_prepare (ctx=0x211430, comp=comp@entry=0x21b054)
      at ../../../../../src/gallium/state_trackers/xa/xa_composite.c:488
  #2  0xb6f454b4 in XAPrepareComposite (op=<optimized out>, pSrcPicture=<optimized out>,
      pMaskPicture=<optimized out>, pDstPicture=<optimized out>, pSrc=0x5b3ad8, pMask=0x0,
      pDst=0x5923b8) at msm-exa-xa.c:533

We can't yet handle solid fill mask, so explicitly reject that, rather
than segfaulting.  Otherwise DDX would need to check XA version to see
if solid fill mask were supported.

Signed-off-by: Rob Clark <robclark@freedesktop.org>