media: hantro: fix post-processing NULL pointer dereference

media: hantro: fix post-processing NULL pointer dereference

The RK3399 variant does not have postproc_regs declared,
this can cause a NULL pointer dereference trying to decode:

[   89.331359] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000

[   89.352804] Call trace:
[   89.353191]  hantro_postproc_disable+0x20/0xe8 [hantro_vpu]
[   89.354056]  hantro_start_prepare_run+0x58/0x68 [hantro_vpu]
[   89.354923]  hantro_h264_dec_prepare_run+0x30/0x6f0 [hantro_vpu]
[   89.355846]  rk3399_vpu_h264_dec_run+0x1c/0x14a8 [hantro_vpu]
[   89.356748]  device_run+0xa4/0xb8 [hantro_vpu]

Fix this by adding a NULL check in hantro_postproc_enable/disable.

Fixes: 8c2d66b036c7 ("media: hantro: Support color conversion via post-processing")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>