drm/i915/guc: stop calling execlists_set_default_submission

Initialize all required entries from guc_set_default_submission, instead
of calling the execlists function. The previously inherited setup has
been copied over from the execlist code and simplified by removing the
execlists submission-specific parts.

v2: move setting of relative_mmio flag to engine_setup_common (Chris)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: John Harrison <john.c.harrison@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210113021236.8164-5-daniele.ceraolospurio@intel.com