gfx: drv: rename PLL/PLL DIV registers according to the TRM

gfx: drv: rename PLL/PLL DIV registers according to the TRM

Rename the PLL and PLL DIV registers according to the TRM and remove the
definitions for the non-existant MRST_FPA1 and MDFLD_DPLL_DIV1.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>