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: 8c2d66b0 ("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>

drivers/staging/media/hantro/hantro_postproc.c

@@ -57,6 +57,9 @@ void hantro_postproc_enable(struct hantro_ctx *ctx)
 	u32 src_pp_fmt, dst_pp_fmt;
 	dma_addr_t dst_dma;
 
+	if (!vpu->variant->postproc_regs)
+		return;
+
 	/* Turn on pipeline mode. Must be done first. */
 	HANTRO_PP_REG_WRITE_S(vpu, pipeline_en, 0x1);
 
@@ -138,5 +141,8 @@ void hantro_postproc_disable(struct hantro_ctx *ctx)
 {
 	struct hantro_dev *vpu = ctx->dev;
 
+	if (!vpu->variant->postproc_regs)
+		return;
+
 	HANTRO_PP_REG_WRITE_S(vpu, pipeline_en, 0x0);
 }