Because some screen not support the alpha visual channel, so the function
'screen.get_rgba_colormap()' will return None, it's a compatible issue, so
change it by another way.
(Bitbake rev: e58ba1ad944ea1ba95a5ada80d4c88f0165bd1ca)
Signed-off-by: Liming An <limingx.l.an@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>