drm/i915/execlists: Prevent merging requests with conflicting flags

We set out-of-bound parameters inside the i915_requests.flags field,
such as disabling preemption or marking the end-of-context. We should
not coalesce consecutive requests if they have differing instructions
as we only inspect the last active request in a context. Thus if we
allow a later request to be merged into the same execution context, it
will mask any of the earlier flags.

References: 2a98f4e6

 ("drm/i915: add infrastructure to hold off preemption on a request")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011190325.10979-9-chris@chris-wilson.co.uk