Steal if from kms_plane.c and put it into igt_fb.h

Also tiny bikeshed to remove the space, so it fits more tidily into
the usual name1=value1, name2=value2 style printing.

v2: Rebase

v3: It better compile :-/

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>