drm/i915: record GEN2_IER in gtier[0] for pre-ilk error capture

In pre-ilk platforms the engine interrupts live in GEN2_IER. Capture it
as part of gtier instead of display.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/f637219fe3accb69963266773b9ef7c1131875e4.1744630147.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>