drm/mipi-dsi: use correct return type for the DSC functions

The functions mipi_dsi_compression_mode() and
mipi_dsi_picture_parameter_set() return 0-or-error rather than a buffer
size. Follow example of other similar MIPI DSI functions and use int
return type instead of size_t.

Fixes: f4dea1aa ("drm/dsi: add helpers for DSI compression mode and PPS packets")
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408-lg-sw43408-panel-v5-2-4e092da22991@linaro.org