drm/xe/irq: Untangle postinstall functions

The xe_irq_postinstall() never actually gets called after installing the
interrupt handler.  This oversight seems to get papered over due to the
fact that the (misnamed) xe_gt_irq_postinstall does more than it really
should and gets called in the middle of the GT initialization.  The
callstack for postinstall is also a bit muddled with top-level device
interrupt enablement happening within platform-specific functions called
from the per-tile xe_gt_irq_postinstall() function.

Clean this all up by adding the missing call to xe_irq_postinstall()
after installing the interrupt handler and pull top-level irq enablement
up to xe_irq_postinstall where we'd expect it to be.

The xe_gt_irq_postinstall() function is still a bit misnamed here; an
upcoming patch will refocus its purpose and rename it.

v2:
 - Squash in patch to actually call xe_irq_postinstall() after
   installing the interrupt handler.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230601215244.678611-25-matthew.d.roper@intel.com


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>