drm/i915: Split execlists/guc reset preparations

drm/i915: Split execlists/guc reset preparations

In the next patch, we will make the execlists reset prepare callback
take into account preemption by flushing the context-switch handler.
This is not applicable to the GuC submission backend, so split the two
into their own backend callbacks.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Michał Winiarski <[email protected]>
CC: Michel Thierry <[email protected]>
Cc: Jeff McGee <[email protected]>
Reviewed-by: Jeff McGee <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]