drm/i915/dg1: Initialize RAWCLK properly

DG1 always uses a 38.4 MHz rawclk rather than the 19.2/24 MHz
frequencies on CNP+.  Note that register bits associated with this
frequency confusingly use 37 for the divider field rather than 38 as you
might expect.

For simplicity, let's just assume that this 38.4 MHz frequency will hold
true for other future platforms with "fake" PCH south displays and that
the CNP-style behavior will remain for other platforms with a real PCH.

Bspec: 49950
Bspec: 49309
Cc: Aditya Swarup <aditya.swarup@intel.com>
Cc: Clinton Taylor <Clinton.A.Taylor@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201007002210.3678024-2-lucas.demarchi@intel.com