drm/i915: Fix the TBT AUX power well enabling

Fix the mapping from a TBT AUX power well index to the DP_AUX_CH_CTL
register.

Fixes: c7375d95 ("drm/i915: Configure AUX_CH_CTL when enabling the AUX power domain")
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628143635.22066-7-imre.deak@intel.com