drm/i915: fix WARNs when reading DDI state while suspended

If runtime PM is enabled and we unset all modes, we will runtime
suspend after __intel_set_mode() , then function
intel_modeset_check_state() will try to read the HW state while it is
suspended and trigger lots of WARNs because it shouldn't be reading
registers.

So on this patch we make intel_ddi_connector_get_hw_state() return
false in case the power domain is disabled, and we also make
intel_display_power_enabled() return false in case the device is
suspended. Notice that we can't just use
intel_display_power_enabled_sw() because while the driver is being
initialized the power domain refcounts are not reflecting the real
state of the hardware.

Just for reference, I have previously published an alternate patch for
this problem, called "drm/i915: get runtime PM at intel_set_mode".

Testcase: igt/pm_pc8
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>