drm/msm: Avoid div-by-zero in dpu_crtc_atomic_check()

The cstate->num_mixers member is only set to a non-zero value once
dpu_encoder_virt_mode_set() is called, but the atomic check function can
be called by userspace before that. Let's avoid the div-by-zero here and
inside _dpu_crtc_setup_lm_bounds() by skipping this part of the atomic
check if dpu_encoder_virt_mode_set() hasn't been called yet. This fixes
an UBSAN warning:

 UBSAN: Undefined behaviour in drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c:860:31
 division by zero
 CPU: 7 PID: 409 Comm: frecon Tainted: G S                5.4.31 #128
 Hardware name: Google Trogdor (rev0) (DT)
 Call trace:
  dump_backtrace+0x0/0x14c
  show_stack+0x20/0x2c
  dump_stack+0xa0/0xd8
  __ubsan_handle_divrem_overflow+0xec/0x110
  dpu_crtc_atomic_check+0x97c/0x9d4
  drm_atomic_helper_check_planes+0x160/0x1c8
  drm_atomic_helper_check+0x54/0xbc
  drm_atomic_check_only+0x6a8/0x880
  drm_atomic_commit+0x20/0x5c
  drm_atomic_helper_set_config+0x98/0xa0
  drm_mode_setcrtc+0x308/0x5dc
  drm_ioctl_kernel+0x9c/0x114
  drm_ioctl+0x2ac/0x4b0
  drm_compat_ioctl+0xe8/0x13c
  __arm64_compat_sys_ioctl+0x184/0x324
  el0_svc_common+0xa4/0x154
  el0_svc_compat_handler+0x

Cc: Abhinav Kumar <abhinavk@codeaurora.org>
Cc: Jeykumar Sankaran <jsanka@codeaurora.org>
Cc: Jordan Crouse <jcrouse@codeaurora.org>
Cc: Sean Paul <seanpaul@chromium.org>
Fixes: 25fdd593 ("drm/msm: Add SDM845 DPU support")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c

@@ -824,7 +824,7 @@ static int dpu_crtc_atomic_check(struct drm_crtc *crtc,
 	struct drm_plane *plane;
 	struct drm_display_mode *mode;
 
-	int cnt = 0, rc = 0, mixer_width, i, z_pos;
+	int cnt = 0, rc = 0, mixer_width = 0, i, z_pos;
 
 	struct dpu_multirect_plane_states multirect_plane[DPU_STAGE_MAX * 2];
 	int multirect_count = 0;
@@ -849,9 +849,11 @@ static int dpu_crtc_atomic_check(struct drm_crtc *crtc,
 
 	memset(pipe_staged, 0, sizeof(pipe_staged));
 
+	if (cstate->num_mixers) {
 		mixer_width = mode->hdisplay / cstate->num_mixers;
 
 		_dpu_crtc_setup_lm_bounds(crtc, state);
+	}
 
 	crtc_rect.x2 = mode->hdisplay;
 	crtc_rect.y2 = mode->vdisplay;