drm/i915/dg1: Enable DPLL for DG1

Add DG1 DPLL Enable register macro and use the macro to enable the
correct DPLL based on PLL id. Although we use
_MG_PLL1_ENABLE/_MG_PLL2_ENABLE these are rather combo phys.

While at it, fix coding style: wrong newlines and use if/else chain

v2: Rewrite original patch from Aditya Swarup based on refactors
upstream

Bspec: 49443, 49206

Cc: Clinton Taylor <Clinton.A.Taylor@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Aditya Swarup <aditya.swarup@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Aditya Swarup <aditya.swarup@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201014191937.1266226-6-lucas.demarchi@intel.com