drm/i915/dp: Check error return during DPCD capability queries

The assumptions of these users of drm_dp_dpcd_readb() is that the passed
in output buffer won't change in case of error, but this isn't
guaranteed. Fix this by treating any error as the lack of the given
capability.

In case of DP_SINK_DEVICE_AUX_FRAME_SYNC_CAP an error would leave the
buffer uninitialized even with the above assumption.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-2-git-send-email-imre.deak@intel.com