lib: extract igt_fb library

lib: extract igt_fb library

Stuff all the framebuffer creation, cairo and drm fourcc format
handling in there. This gives us a very clear cut in the headers where
igt_fb.c only needs to include igt_fb.h, and igt_kms.c includes both
(well igt_kms.h pulls in igt_fb.h since we always need this).

The aim here is to add api docs for igt_fb since that part of the kms
library seems fairly stable already, while all the mode setting and
iteration is still a bit in flux.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>