fbdev: fix pixman compile on old pixman

My QEMU compile failed with the following error:

qemu-pixman.c: In function ‘qemu_pixman_get_type’:
qemu-pixman.c:24: error: ‘PIXMAN_TYPE_BGRA’ undeclared (first use in this function)
qemu-pixman.c:24: error: (Each undeclared identifier is reported only once
qemu-pixman.c:24: error: for each function it appears in.)

Guard the PIXMAN_TYPE_BGRA branch like in the case right above the failing
case, so that compilation is fixed. Functionality on such old pixman is a
different question ;-).

Signed-off-by: Alexander Graf <agraf@suse.de>
