Instead of passing around a boatload of integers everywhere let's
just pass around the igt_fb struct. That obviously means we have to
populate it first sufficiently, to which end we'll add a small helper.
Later on the stride/size calculations will consult the already
pre-populated igt_fb and fill in the rest as needed.

This makes the whole thing a lot less error prone as it's impossible
to accidentally pass the arguments in the wrong order when there's
just the one of them, and it's a pointer.

v2: Rebase due to uint64_t size

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>