drm/i915/tgl: Add support for dkl pll write

Add a new function to write to dkl phy pll registers. As per the
bspec all the registers are read modify write.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190924210040.142075-2-jose.souza@intel.com