exa: Use PictureMatchFormat for source-only picture format description

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

Fixes: 8171d4c2 "render: Store and use all 16bpc of precision for
                     solid pixels (v2.1)"
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 3ebef6ab)