I recently had this discussion with Daniel where I didn't want to use
igt_drm_format_to_bpp() because it uses the format_desc array, and
igt_fb currently assumes that all the format_desc formats have a
matching valid Cairo format, so I wouldn't be able to easily add
formats such as ARGB2101010.

The function that has the assumption mentioned above is
igt_get_all_formats: its current users call igt_get_all_formats, and
then call cairo-dependent functions, such as igt_get_cairo_ctx on the
returned formats.

In order to document the current behavior and prevent any problems in
case we start adding new formats without matching Cairo versions to
format_desc, rename igt_get_all_formats to igt_get_all_cairo_formats
and make it explicitly check for CAIRO_FORMAT_INVALID.

Requested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>