drm/i915/adl_p: Add PLL Support

The clocks in ALD_P is similar to that of TGL.
The combo PLLs  use the same DPLL0, DPLL1 and TBT_PLL.

This patch adds the helper function intel_mg_pll_enable_reg()
which is similar to intel_combo_pll_enable_reg() for being lookup
place for PLL_ENABLE register in combo phy cases.

Bspec: 55409,55316
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Clinton Taylor <clinton.a.taylor@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Clinton Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210519000625.3184321-15-lucas.demarchi@intel.com