drm/i915: Assert that VED and ISP are power gated

drm/i915: Assert that VED and ISP are power gated

As there are no upstream drivers for VED or ISP let's just
assert that they are power gated. Otherwise they would
prevent s0ix entry.

For ISP this is only relevant when it is not exposed as a
PCI device and instead is a subordinate of the gunit. When
exposed as a PCI device it will be handled by the
atomisp2_pm driver.

On my VLV FFRD8 board the firmware power gates both of these
by default. Let's assume that is always the case and just
WARN if we ever encounter something different.

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129175504.3630-2-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>