drm/msm/dpu: retrieve DSI DSC struct through priv->dsi[0]

Currently struct drm_dsc_config for DSI is populated at display
setup during system boot up. This mechanism works fine with
embedded display but not for pluggable displays as the
struct drm_dsc_config will become stale once external display
is unplugged.

In preparation of adding support for DP DSC support
move storing of DSI DSC struct to atomic_enable() so that same
mechanism will work for both embedded display and pluggable
displays.

Changes in v4:
-- fix checkpatch.pl warning

Changes in v5:
-- delete dpu_encoder_get_dsc_config() from atomic_mode_set

Signed-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/543867/
Link: https://lore.kernel.org/r/1687454686-10340-2-git-send-email-quic_khsieh@quicinc.com


[DB: fixed commit message]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>