Set color_depth according to connector->bpc.

Changes since v1:
 - Fixed min_bpc is effectively set but not used in
komeda_crtc_get_color_config().

Changes since v2:
 - Align the code.
Signed-off-by: Lowry Li (Arm Technology China) <lowry.li@arm.com>
Reviewed-by: Mihail Atanassov <mihail.atanassov@arm.com>
Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191012065030.12691-1-lowry.li@arm.com