drm/i915/irq: Remove duplicated irq_enabled variable

drm/i915/irq: Remove duplicated irq_enabled variable

Let's kill this legacy iand almost unused rq_enabled version
in favor of the real one that is checked at
intel_irqs_enabled().

The commit 'ac1723c16b66 ("drm/i915: Track IRQ state
in local device state")' shows that this was a legacy
DRM level irq_enabled information that got removed.

But the driver one already existed under a different
name.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240912172539.418957-1-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>