fb: Fix invalid bpp for 24bit depth window

We have a hack in fb layer for a 24bpp screen to use 32bpp images, and
fbCreateWindow() replaces its drawable.bitsPerPixel field
appropriately.  But, the problem is that it always replaces when 32bpp
is passed.  If the depth is 32, this results in bpp < depth, which is
actually invalid.

Meanwhile, fbCreatePixmap() has a more check and it creates with 24bpp
only when the passed depth <= 24 for avoiding such a problem.

This oneliner patch just adds the similar check in fbCreateWindow().
This (hopefully) fixes the long-standing broken graphics mess of
cirrus KMS with 24bpp.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit fe5018e0564118a7a8198fa286186fdb9ed818c7)
Signed-off-by: Julien Cristau <jcristau@debian.org>