drm/xe: Replace xe_gt_irq_postinstall with xe_irq_enable_hwe

drm/xe: Replace xe_gt_irq_postinstall with xe_irq_enable_hwe

The majority of xe_gt_irq_postinstall() is really focused on the
hardware engine interrupts; other GT-related interrupts such as the GuC
are enabled/disabled independently.  Renaming the function and making it
truly GT-specific will make it more clear what the intended focus is.

Disabling/masking of other interrupts (such as GuC interrupts) is
unnecessary since that has already happened during the irq_reset stage,
and doing so will become harmful once the media GT is re-enabled since
calls to xe_gt_irq_postinstall during media GT initialization would
incorrectly disable the primary GT's GuC interrupts.

Also, since this function is called from gt_fw_domain_init(), it's not
necessary to also call it earlier during xe_irq_postinstall; just
xe_irq_resume to handle runtime resume should be sufficient.

v2:
 - Drop unnecessary !gt check.  (Lucas)
 - Reword some comments about enable/unmask for clarity.  (Lucas)

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230601215244.678611-26-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>