drm/i915/cnl: Fix aux selection for WA 1178

Current code always select _CNL_AUX_ANAOVRD1_B
register regardless the pw in use.

CNL_DISP_PW_AUX_B = 9
CNL_DISP_PW_AUX_C = 10
CNL_DISP_PW_AUX_D = 11

And for pick we want

B = 0
C = 1
D = 2

Fixes: ddd39e4b ("drm/i915/cnl: apply Display WA #1178 to fix type C dongles")
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180123215245.24026-1-rodrigo.vivi@intel.com