[media] coda: use ctx->fh.m2m_ctx instead of ctx->m2m_ctx

v4l2_fh already contains a mem2mem context pointer. Use it.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>