drm/i915: opportunistically apply ALLOC_CONTIGIOUS

It looks like this code was accidentally dropped at some point(in a
slightly different form), so add it back. The gist is that if we know
the allocation will be one single chunk, then we can just annotate the
BO with I915_BO_ALLOC_CONTIGUOUS, even if the user doesn't bother. In
the future this should allow us to avoid using vmap for such objects,
in some upcoming patches.

v2(Thomas):
  - Tweak the commit message to mention the future motivation

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220202173154.3758970-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_region.c

@@ -62,6 +62,9 @@ i915_gem_object_create_region(struct intel_memory_region *mem,
 
 	size = round_up(size, default_page_size);
 
+	if (default_page_size == size)
+		flags |= I915_BO_ALLOC_CONTIGUOUS;
+
 	GEM_BUG_ON(!size);
 	GEM_BUG_ON(!IS_ALIGNED(size, I915_GTT_MIN_ALIGNMENT));