modetest: initialize handles/pitches in set_plane()

Only the 'offsets' array was initialized to zero.
Since bo_create only sets the handles which are
necessary, were we passing garbage data to the
kernel when calling drmModeAddFB2 later.

The issue only seems to appear when passing e.g.
NV12 data to the kernel, a case where not only
handles[0] is used. I therefore also removed the
corresponding comment.

v2: Do the same for set_mode(), set_cursors()
    and test_page_flip().

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>