drm/i915: Select DPLL's via mask

This slightly simplifies the EHL DPLL4 handling and also gives us more
flexibility in the future in case we need to skip the use of specific
PLL's (e.g., due to hardware workarounds and such).

v2:
 - Replace GENMASK() with or'd BIT()'s to make the specific DPLLs more
   explicit.  (Ville)
 - s/unsigned/unsigned long/.  (Lucas)

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191008172920.11362-1-matthew.d.roper@intel.com