media: ti-vpe: vpe: fix a v4l2-compliance failure causing a kernel panic

v4l2-compliance fails with this message:

   warn: v4l2-test-formats.cpp(717): \
   	TRY_FMT cannot handle an invalid pixelformat.
   test VIDIOC_TRY_FMT: FAIL

This causes the following kernel panic:

Unable to handle kernel paging request at virtual address 56595561
pgd = ecd80e00
*pgd=00000000
Internal error: Oops: 205 [#1] PREEMPT SMP ARM
...
CPU: 0 PID: 930 Comm: v4l2-compliance Not tainted \
	4.14.62-01715-gc8cd67f49a19 #1
Hardware name: Generic DRA72X (Flattened Device Tree)
task: ece44d80 task.stack: ecc6e000
PC is at __vpe_try_fmt+0x18c/0x2a8 [ti_vpe]
LR is at 0x8

Because the driver fails to properly check the 'num_planes' values for
proper ranges it ends up accessing out of bound data causing the kernel
panic.

Since this driver only handle single or dual plane pixel format, make
sure the provided value does not exceed 2 planes.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>