drm/i915: turn the eDP VDD on for any i2c transactions

If the eDP output is disabled, then we try to use /dev/i2c-X file to
do i2c transations, we get a WARN from intel_dp_check_edp() saying
we're trying to do AUX communication with the panel off. So this
commit reorganizes the code so we enable the VDD at
intel_dp_i2c_aux_ch() instead of just the callers inside i915.ko.

This fixes the i2c subtest from the pc8 test of intel-gpu-tools on
machines that have eDP panels.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>