drm/vc4: crtc: Add null pointer error handling

drm/vc4: crtc: Add null pointer error handling

Crash occurs when vc4_encoder is null. Add null error handling

  Unable to handle kernel NULL pointer dereference at virtual address 0000000000000090
  Mem abort info:
    ESR = 0x96000005
    EC = 0x25: DABT (current EL), IL = 32 bits
    SET = 0, FnV = 0
    EA = 0, S1PTW = 0
  Data abort info:
    ISV = 0, ISS = 0x00000005
    CM = 0, WnR = 0
  user pgtable: 4k pages, 39-bit VAs, pgdp=00000000f2a96000
  [0000000000000090] pgd=0000000000000000, pud=0000000000000000
  Internal error: Oops: 96000005 [#1] PREEMPT SMP
  Modules linked in: brcmfmac joydev brcmutil rpivid_mem
  CPU: 0 PID: 32 Comm: kworker/0:1 Not tainted 5.4.50-v8+ #414
  Hardware name: Raspberry Pi 4 Model B (DT)
  Workqueue: events output_poll_execute
  pstate: 80000005 (Nzcv daif -PAN -UAO)
  pc : vc4_crtc_atomic_disable+0x150/0x2e8
  lr : vc4_crtc_atomic_disable+0x50/0x2e8
  ...
  Call trace:
   vc4_crtc_atomic_disable+0x150/0x2e8
   drm_atomic_helper_commit_modeset_disables+0x344/0x410
   vc4_atomic_complete_commit+0xd4/0x530
   vc4_atomic_commit+0xf4/0x190
   drm_atomic_commit+0x54/0x60
   ...

Change-Id: Ib3271bb3c8318079fe5815711f89a217e835adc6
Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>