drm/i915/migrate: move the sanity check

Move the sanity check that both src and dst are never both system
memory, which should never happen on discrete, and likely means we have
a bug. The only exception is on integrated where we trigger this path in
the selftests.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220324172143.377104-2-matthew.auld@intel.com

drivers/gpu/drm/i915/gt/intel_migrate.c

@@ -530,6 +530,7 @@ intel_context_migrate_copy(struct intel_context *ce,
 	int err;
 
 	GEM_BUG_ON(ce->vm != ce->engine->gt->migrate.context->vm);
+	GEM_BUG_ON(IS_DGFX(ce->engine->i915) && (!src_is_lmem && !dst_is_lmem));
 	*out = NULL;
 
 	GEM_BUG_ON(ce->ring->size < SZ_64K);
@@ -566,8 +567,6 @@ intel_context_migrate_copy(struct intel_context *ce,
 		src_offset = 0;
 		dst_offset = CHUNK_SZ;
 		if (HAS_64K_PAGES(ce->engine->i915)) {
-			GEM_BUG_ON(!src_is_lmem && !dst_is_lmem);
-
 			src_offset = 0;
 			dst_offset = 0;
 			if (src_is_lmem)