drm/i915/irq: move i915->irq_lock to display->irq.lock

drm/i915/irq: move i915->irq_lock to display->irq.lock

Observe that i915->irq_lock is no longer used to protect anything
outside of display. Make it a display thing.

This allows us to remove the ugly #define irq_lock irq.lock hack from xe
compat header.

Note that this is slightly more subtle than it first looks. For i915,
there's no functional change here. The lock is moved. However, for xe,
we'll now have *two* locks, xe->irq.lock and display->irq.lock. These
should protect different things, though. Indeed, nesting in the past
would've lead to a deadlock because they were the same lock.

With the i915 references gone, we can make a handful more files
independent of i915_drv.h.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/6d8d2ce0f34a9c7361a5e2fcf96bb32a34c57e76.1746536745.git.jani.nikula@intel.com
[Jani: Fixed a comment while applying.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>