gfx: display: allow powering down GL3 cache regardless of GL3 config

gfx: display: allow powering down GL3 cache regardless of GL3 config

The commit "psb_video: remove OSPM_GL3_CACHE_ISLAND when not enabled" made
it impossible to power down the GL3 cache island when
CONFIG_MDFD_GL3=n. It's powered on at boot (likely by firmware) so allow
powering down regardless of config.

This fixes the following situation in /sys/kernel/debug/mid_pmu_states when
display is off:

pci 8086 0130 0000:00:02.0             pvrsrvkm:
                      |------->             GFX                   D0i3
                      |------->   Video Decoder                   D0i3
                      |------->   Video Encoder                   D0i3
                      |------->       GL3 Cache                   D0
                      |------->       Display A                   D0i3
                      |------->       Display B                   D0i3
                      |------->       Display C                   D0i3
                      |------->        MIPI-DSI                   D0i3

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>