drm/i915/ehl: Ensure that the DDI selection MUX is programmed correctly

Perhaps in some cases the BIOS/GOP or other firmware may turn on
PHY A but may not program the MUX correctly. Therefore, re-program
PHY A if it is determined after reading the VBT that the value
programmed for the MUX bit does not match the expected value.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121235848.8457-1-vivek.kasireddy@intel.com