Ever since commit 3fa65f4b

 ("fb: Add support for conversions
through pixman") we can generate a valid cairo surface for any plane,
use this to avoid having to implement our own conversion routine.

Instead of duplicating this functionality in igt_fb_convert_with_stride,
we can simply convert this to a few cairo calls, because we now support
cairo calls to any of the supported framebuffer formats.

This is required to make this function more generic, and convert from any
format/modifier to any other format/modifier.

Changes since v1:
- Return fb_id in the cairo case.
Changes since v2:
- Remove the manual conversion fallback.
Changes since v3:
- Integrate VC4 conversion routines.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>