drm/i915/execlists: Remove the startup spam

Execlists is now enabled by default and included in the list of
capabilities printed out to dmesg and beyond. We do not need to mention
it again, every time we restart the engine, so kill the spam.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205092201.19476-6-chris@chris-wilson.co.uk