drm/i915/icl: Do read-modify-write as needed during MG PLL programming

Some MG PLL registers have fields that need to be preserved at their HW
default or BIOS programmed values. So make sure we preserve them.

v2:
- Add comment to icl_mg_pll_write() explaining the need for register
  masks. (Vandita)
- Fix patchwork checkpatch warning.

v3:
- Rebase on drm-tip.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com> (v1)
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180619164115.7835-1-imre.deak@intel.com