drm/exynos: Check for NULL dereference of crtc

drm/exynos: Check for NULL dereference of crtc

The commit "drm/exynos: remove exynos_plane_dpms" (d9ea6256) removed the
use of the enabled flag, which means that the code may attempt to call
win_enable on a NULL crtc. This results in the following oops on
Arndale:

[    1.673479] Unable to handle kernel NULL pointer dereference at virtual address 00000368
[    1.681500] pgd = c0004000
[    1.684154] [00000368] *pgd=00000000
[    1.687713] Internal error: Oops: 5 [#1] PREEMPT SMP ARM
[    1.693012] Modules linked in:
[    1.696045] CPU: 1 PID: 1 Comm: swapper/0 Not tainted
3.19.0-07545-g57485fa #1907
[    1.703524] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
(....)
[    2.014803] [<c02f9cfc>] (exynos_plane_destroy) from [<c02e61b4>] (drm_mode_config_cleanup+0x168/0x20c)
[    2.024178] [<c02e61b4>] (drm_mode_config_cleanup) from [<c02f66fc>] (exynos_drm_load+0xac/0x12c)

This patch adds in a check to ensure exynos_crtc is not NULL before it
is dereferenced.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>