drm/i915/dp: Remove debug dependency of DPCD SW/HW revision read

Performing DPCD AUX reads based on debug settings may introduce obscure
bugs in other places that depend on the read being done (or being not
done). To reduce the uncertainty perform the reads unconditionally.

Cc: Mika Kahola <mika.kahola@intel.com>
Suggested-by: Jani Nikula <jani.nikula@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/1477326811-30431-3-git-send-email-imre.deak@intel.com