drm/i915/ehl/dsi: Enable AFE over PPI strap

The other additional step in the DSI sequence for EHL.

v2:
- Using REG_BIT()(Matt)
- Fixed commit message typo(Vandita)

BSpec: 20597
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190619233134.20009-2-jose.souza@intel.com