[media] s5p-fimc: Do not attempt to disable not enabled media pipeline

This fixes following warnings when all links are being disconnected:
[   20.080000] WARNING: at drivers/media/platform/s5p-fimc/fimc-mdevice.c:1269 __fimc_md_set_camclk+0x208/0x20c()
[   20.090000] Modules linked in:
[   20.095000] [<c001603c>] (unwind_backtrace+0x0/0x13c) from [<c00246dc>] (warn_slowpath_common+0x54/0x64)
[   20.105000] [<c00246dc>] (warn_slowpath_common+0x54/0x64) from [<c0024708>] (warn_slowpath_null+0x1c/0x24)
[   20.115000] [<c0024708>] (warn_slowpath_null+0x1c/0x24) from [<c0323fe8>] (__fimc_md_set_camclk+0x208/0x20c)
[   20.125000] [<c0323fe8>] (__fimc_md_set_camclk+0x208/0x20c) from [<c0324368>] (__fimc_pipeline_close+0x38/0x48)
[   20.135000] [<c0324368>] (__fimc_pipeline_close+0x38/0x48) from [<c0325848>] (fimc_md_link_notify+0x10c/0x198)
[   20.145000] [<c0325848>] (fimc_md_link_notify+0x10c/0x198) from [<c02f9dd4>] (__media_entity_setup_link+0x1c0/0x1e8)
[   20.155000] [<c02f9dd4>] (__media_entity_setup_link+0x1c0/0x1e8) from [<c02f9710>] (media_device_ioctl+0x2c0/0x41c)
[   20.165000] [<c02f9710>] (media_device_ioctl+0x2c0/0x41c) from [<c02f9938>] (media_ioctl+0x30/0x34)
[   20.175000] [<c02f9938>] (media_ioctl+0x30/0x34) from [<c00cf0bc>] (do_vfs_ioctl+0x84/0x5e8)
[   20.185000] [<c00cf0bc>] (do_vfs_ioctl+0x84/0x5e8) from [<c00cf65c>] (sys_ioctl+0x3c/0x60)
[   20.190000] [<c00cf65c>] (sys_ioctl+0x3c/0x60) from [<c000f040>] (ret_fast_syscall+0x0/0x30)

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>