lib/fb: Fix rgb24 to nv12 conversion

A typo fix in 1x2 pixel block conversion code, revealed by GCC 9

Fixes: 1c7ef389 ("lib: Use igt_matrix for ycbcr<->rgb conversion")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109257


Reported-by: Martin Liska <mliska@suse.cz>
Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>