drm/i915: Remove intel_psr_is_enabled function.

This function was in use to check if PSR feature got enabled.
However on HSW and BDW we currently force psr exit by disabling
EDP_PSR_ENABLE bit at EDP_PSR_CTL(dev). So this function was actually
returning the active/inactive state that is different from the enable/disable
meaning and had the risk of false negative.

But anyway this check with DRRS was dangerous, since DRRS could try to get enabled
before PSR gets there. So let's just remove it for now.
A proper synchronization mechanism must be implemented later probably
using pipe config.

Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>