drm/i915/selftests: Split igt_atomic_reset testcase

Split igt_atomic_reset selftests into separate full & engines parts,
so we can move former to the dedicated reset selftests file.

While here change engines test to loop first over atomic phases and
then loop over available engines.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190522193203.23932-3-michal.wajdeczko@intel.com