drm/msm: Do not run snapshot on non-DPU devices

Since commit 98659487 ("drm/msm: add support to take dpu snapshot")
the following NULL pointer dereference is seen on i.MX53:

[ 3.275493] msm msm: bound 30000000.gpu (ops a3xx_ops)
[ 3.287174] [drm] Initialized msm 1.8.0 20130625 for msm on minor 0
[ 3.293915] 8<--- cut here ---
[ 3.297012] Unable to handle kernel NULL pointer dereference at virtual address 00000028
[ 3.305244] pgd = (ptrval)
[ 3.307989] [00000028] *pgd=00000000
[ 3.311624] Internal error: Oops: 805 [#1] SMP ARM
[ 3.316430] Modules linked in:
[ 3.319503] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.14.0+g682d702b426b #1
[ 3.326652] Hardware name: Freescale i.MX53 (Device Tree Support)
[ 3.332754] PC is at __mutex_init+0x14/0x54
[ 3.336969] LR is at msm_disp_snapshot_init+0x24/0xa0

i.MX53 does not use the DPU controller.

Fix the problem by only calling msm_disp_snapshot_init() on platforms that
use the DPU controller.

Cc: stable@vger.kernel.org
Fixes: 98659487

 ("drm/msm: add support to take dpu snapshot")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210914174831.2044420-1-festevam@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>