lib/igt_fb: switch to igt_ prefix from kmstest_

lib/igt_fb: switch to igt_ prefix from kmstest_

Shorter and more in line with our general igt_ prefix for everything
which isn't somehow intel or i915-gem or otherwise hw specific - these
helpers here are all fully generic framebuffer handling functions
based on kms + cairo.

Well, the actual buffer alloc is done with i915 gem, but meh ;-)

Two special cases:
- bpp_depth_to_drm_format and drm_format_to_bpp completely lacked
  prefixes, so just add igt_.
- write_fb was a bit misleading given that we have gem_write for
  uploading to buffers. Rename that to write_fb_to_png to make it
  crystal clear what this thing does even without looking at docs.

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