drm/rockchip/dsi: fix handling mipi_dsi_pixel_format_to_bpp result

The function can return negative value so it should be assigned to signed
variable.

The problem has been detected using patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Chris Zhong <zyw@rock-chips.com>