drm/i915: Update bits per component for display info

DisplayPort branch device may define max supported bits per
component. Update display info based on this value if bpc
is defined.

v2: cleanup to match the drm_dp_helper.c patches introduced
    earlier in this series
v3: Fill bpc for connector's display info in separate
    drm_dp_helper function (Daniel)
v4: remove updating bpc for display info as it may be overridden
    when parsing EDID. Instead, check bpc for DP branch device
    during compute_config
v5: Indentation fixes (Jim Bride)

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473419458-17080-11-git-send-email-mika.kahola@intel.com