drm/i915: Remove redundant return value and WARN_ON

execlists_submit_context() always returns 0, which is redundant.
And its name is inaccurate, since it actually submits (up to)
TWO contextS. So we rename it, change it to "void", and remove
the WARN_ON() testing its return value.

Change-Id: Ie225b0eca7754c6093c8b8bd15550b251b6feb82
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>