gfx: drv: clean up the save/restore of the fp/mipi/dpll regs

gfx: drv: clean up the save/restore of the fp/mipi/dpll regs

We have an fp and dpll reg for pipe A and B. Pipe C shares pipe A's fp
and dpll setting, since the two pipes use the same clock setting. Store
these regs into the pipe_regs array instead of having separate names for
them. Pipe C's storage space will be wasted.

Pipe A and C has a MIPI control reg, store these into the pipe_regs
array. Pipe B's storage space will be wasted.

There is 12 bytes wasted, we pay this for a cleaner abstraction.

The original code saved the same dpll,fp regs twice (for pipe A and C)
now avoid saving/restoring them for pipe C. This may also avoid glitches
on the screen as we momentarily disabled pipe A's clock while it was
already setup and active.

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>