drm/i915/guc: enable guc interrupts unconditionally in uc_resume

Probably lost while rebasing commit eacd8391 ("drm/i915/guc: Keep GuC
interrupts enabled when using GuC").

Not really needed since i915_gem_init_hw is called before uc_resume, but
it brings symmetry to uc_suspend.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328205851.16188-1-michel.thierry@intel.com