exa: Use PictureMatchFormat for source-only picture format description

Their pFormat member is NULL, which resulted in a crash in
miRenderColorToPixel.

Fixes: 8171d4c2d6 "render: Store and use all 16bpc of precision for
                     solid pixels (v2.1)"
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 3ebef6ab85)

exa/exa_render.c

@@ -291,7 +291,8 @@ exaTryDriverSolidFill(PicturePtr pSrc,
         pixel = exaGetPixmapFirstPixel(pSrcPix);
     }
     else
-        miRenderColorToPixel(pSrc->pFormat,
+        miRenderColorToPixel(PictureMatchFormat(pDst->pDrawable->pScreen, 32,
+                                                pSrc->format),
                              &pSrc->pSourcePict->solidFill.fullcolor,
                              &pixel);