Add const qualifier to arguments of drmModeAddFB2()

Both drmModeAddFB2() and drmModeAddFB2WithModifiers() have some
arguments that are just pointers to uint32_t in disguise. These
are not modified (just copied) in the function, so we can add a
const qualifier here.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>