drm/i915: Allow "max bpc" property to limit pipe_bpp

Use the newly added "max bpc" connector property to limit pipe bpp.

V3: Use drm_connector_state to access the "max bpc" property
V4: Initialize the drm property, add suuport to DP(Ville)
V5: Use the property in the connector and fix CI failure(Ville)
V6: Use the core function to attach max_bpc property, remove the redundant
    clamping of pipe bpp based on connector info
V7: Fix Checkpatch warnings
V9: Cleanup connected_sink_max_bpp and fix initial value in DP(Ville)
V12: Fix debug message(Ville)
V13: Remove the redundant check and simplify the check logic(Stan)
V14: Fix the check in connected_sink_max_bpp(Stan)
v15 (From Manasi): Add missing break (Stan)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Kishore Kadiyala <kishore.kadiyala@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181023014400.16055-1-manasi.d.navare@intel.com