drm/i915/lspcon: Fail LSPCON probe if the start of DPCD can't be read

All types of DP devices (eDP, DP sink, DP branch) will fail their probe
if the start of DPCD can't be read. The LSPCON PCON functionality also
depends on accessing this area, so fail the probe if the read fails.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477326811-30431-6-git-send-email-imre.deak@intel.com