drm/xe: proper setting of irq enabled flag

IRQ enabled flag should be set only after request irq succeeds.

Reviewed-by: Ohad Sharabi <osharabi@habana.ai>
Signed-off-by: Dani Liberman <dliberman@habana.ai>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>