Fix use of uninitialized fields reported by valgrind

Fix use of uninitialized fields reported by valgrind

In pixman-noop.c and pixman-sse2.c, we are accessing
image->bits.width/height without first making sure the image is a bits
image. The warning is harmless because we never act on this
information without checking that the image is a8r8g8b8, but valgrind
does warn about it.

In pixman-noop.c, just reorder the clauses in the if statement; in
pixman-sse2.c require images to have the FAST_PATH_BITS_IMAGE flag
set.