gfx: display: switch to the power island management code in the PMU driver

gfx: display: switch to the power island management code in the PMU driver

Both the GFX and ISP drivers need to switch on and off the relevant power
islands through PUnit. If a centralized driver with proper locking is not
used, they may stomp on each others' toes accessing the PUnit registers.

Obviously, this also reduces code duplication, and the power island
management code is of better quality in the PMU driver anyway.

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