We were supporting 24bpp.  However, the pixel organisation in
memory was 0RGB, so it was 24bpp in 32bit words.  This means
we're actually supporting 32bpp and not 24bpp.

Also, add a check to ensure that we don't exceed the available
framebuffer when changing display resolutions.
Signed-off-by: Russell King <rmk@arm.linux.org.uk>