drm/msm/dpu: don't try using 2 LMs if only one DSC is available

[ Upstream commit b9699dd ]

Current topology code will try using 2 LMs with just one DSC, which
breaks cases like SC7280 / Fairphone5. Forbid using 2 LMs split in such
a case.

Fixes: 1ce69c2 ("drm/msm/dpu: move resource allocation to CRTC")
Reported-by: Luca Weiss <luca.weiss@fairphone.com>
Closes: https://lore.kernel.org/r/DH1IKLU0YZYU.2SW4WYO7H3H4R@fairphone.com/
Tested-by: Luca Weiss <luca.weiss@fairphone.com> # qcm6490-fairphone-fp5
Patchwork: https://patchwork.freedesktop.org/patch/712386/
Link: https://lore.kernel.org/r/20260317-fix-3d-dsc-v1-1-88b54f62f659@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: lumag

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

@@ -1410,7 +1410,8 @@ static struct msm_display_topology dpu_crtc_get_topology(
 		topology.num_lm = 2;
 	else if (topology.num_dsc == 2)
 		topology.num_lm = 2;
-	else if (dpu_kms->catalog->caps->has_3d_merge)
+	else if (dpu_kms->catalog->caps->has_3d_merge &&
+		 topology.num_dsc == 0)
 		topology.num_lm = (mode->hdisplay > MAX_HDISPLAY_SPLIT) ? 2 : 1;
 	else
 		topology.num_lm = 1;