drm/msm/dpu: Require linear modifier for writeback framebuffers

drm/msm/dpu: Require linear modifier for writeback framebuffers

UBWC-related register configuration for writeback is not implemented in
the driver yet but there aren't any checks for non-linear modifiers in
atomic_check. Thus when compressed framebuffer is attached to writeback
connector it will be filled with linear image data. This patch forbids
non-linear modifiers for writeback framebuffers until UBWC support for
writeback is properly implemented.

Fixes: 71174f362d67 ("drm/msm/dpu: move writeback's atomic_check to dpu_writeback.c")
Signed-off-by: Vladimir Lypak <vladimir.lypak@gmail.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/681922/
Link: https://lore.kernel.org/r/20251017-b4-dpu-fixes-v1-5-40ce5993eeb6@gmail.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>