If s5p_mfc_release() runs after s5p_mfc_remove(), the former will access
invalid s5p_mfc_dev pointer saved in the s5p_mfc_ctx and runs into kernel
paging request errors.

Clear ctx dev pointer in s5p_mfc_remove() and change s5p_mfc_release() to
avoid work that requires ctx->dev.

odroid kernel: Unable to handle kernel paging request at virtual address
    f17c1104
odroid kernel: pgd = ebca4000
odroid kernel: [f17c1104] *pgd=6e23d811, *pte=00000000, *ppte=00000000
odroid kernel: Internal error: Oops: 807 [#1] PREEMPT SMP ARM
odroid kernel: Modules linked in: cpufreq_userspace cpufreq_powersave
    cpufreq_conservative s5p_mfc s5p_jpeg v4l2_mem2mem
    videobuf2_dma_contig videobuf2_memops videobuf2_v4l2 videobuf2_core
    v4l2_common videodev media
odroid kernel: Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
odroid kernel: task: c2241400 ti: e7018000 task.ti: e7018000
odroid kernel: PC is at s5p_mfc_reset+0x40/0x28c [s5p_mfc]
odroid kernel: LR is at s5p_mfc_reset+0x34/0x28c [s5p_mfc]
odroid kernel: pc : [<bf15bfbc>]    lr : [<bf15bfb0>] psr: 60010013
odroid kernel: [<bf15bfbc>] (s5p_mfc_reset [s5p_mfc]) from [<bf15c62c>]
    (s5p_mfc_deinit_hw+0x14/0x3c [s5p_mfc])
odroid kernel: [<bf15c62c>] (s5p_mfc_deinit_hw [s5p_mfc]) from [<bf155958>]
    (s5p_mfc_release+0xac/0x1c4 [s5p_mfc])
odroid kernel: [<bf155958>] (s5p_mfc_release [s5p_mfc]) from [<bf021344>]
    (v4l2_release+0x38/0x74 [videodev])
odroid kernel: [<bf021344>] (v4l2_release [videodev]) from [<c01e4274>]
    (__fput+0x80/0x1c8)
odroid kernel: [<c01e4274>] (__fput) from [<c0135c58>]
    (task_work_run+0x94/0xc8)
odroid kernel: [<c0135c58>] (task_work_run) from [<c010a9d4>]
    (do_work_pending+0x7c/0xa4)
odroid kernel: [<c010a9d4>] (do_work_pending) from [<c0107794>]
    (slow_work_pending+0xc/0x20)
odroid kernel: Code: eb3edacc e5953078 e3a06000 e2833c11 (e5836004)
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Tested-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>