drm/{i915, xe}/display: duplicate gen2 irq/error init/reset in display irq

drm/{i915, xe}/display: duplicate gen2 irq/error init/reset in display irq

Duplicate gen2_irq_reset(), gen2_assert_iir_is_zero(), gen2_irq_init(),
gen2_error_reset(), and gen2_error_init() in intel_display_irq.c.

This allows us to drop the duplicates from xe, and prepares for future
cleanups. Although duplication is undesirable in general, in this case
the local duplicates lead to a cleaner end result.

There's a slight wrinkle in gen2_assert_iir_is_zero(). We need to use
non-device based logging until we pass in struct intel_display in a
separate change.

v2:
- Keep xe compat stuff due to series reorder and rebase
- Keep the WARN as regular WARN
- Rename the functions in the same go

Suggested-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/296d74731cce57ab7534c57969d3146294adda57.1763370931.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>