drm/xe/gt: Enable interrupt while initializing root gt

At present the interrupts are enabled while initializing the last GT.
But this is incorrect for a Multi-GT platform, as root GT initialization
will fail with interrupt disabled. Interrupts are required for
the GuC submission triggered during initialization.
Enable the interrupt during the root GT initialization.

Signed-off-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>