drm/i915/execlists: Refactor out can_merge_rq()

In the next patch, we add another user that wants to check whether
requests can be merge into a single HW execution, and in the future we
want to add more conditions under which requests from the same context
cannot be merge. In preparation, extract out can_merge_rq().

v2: Reorder tests to decide if we can continue filling ELSP and bonus
comments.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208235108.23127-1-chris@chris-wilson.co.uk