drm/i915/tgl: Use refclk/2 as bypass frequency

drm/i915/tgl: Use refclk/2 as bypass frequency

Unlike gen11, which always ran at 50MHz when the cdclk PLL was disabled,
TGL runs at refclk/2.  The 50MHz croclk/2 is only used by hardware
during some power state transitions.

Bspec: 49201
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190905181337.23727-1-matthew.d.roper@intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>