drm/i915/pll: Centralize PLL_ENABLE register lookup

We currenty check for platform at multiple parts in the driver
to grab the correct PLL. Let us begin to centralize it through a
helper function.

v2: s/intel_get_pll_enable_reg()/intel_combo_pll_enable_reg() (Ville)

v3: Clean up combo_pll_disable() (Rodrigo)

v4: s/dev_priv/i915 (Jani)
Move static and return type to the same line( Ville, Jani)

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200914175703.15024-1-anusha.srivatsa@intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>