drm/i915: Remove redundant check on execlists interrupt

Since commit 4a118ecb ("drm/i915: Filter out spurious execlists
context-switch interrupts") we probe execlists->active, and no longer
have to peek at the execlist interrupt to determine if the tasklet still
needs to be run to drain the ELSP.

References: 4a118ecb ("drm/i915: Filter out spurious execlists context-switch interrupts")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208151224.16285-1-chris@chris-wilson.co.uk