drm/i915/gem: Call i915_gem_flush_free_objects() in i915_gem_dumb_create()

This doesn't really fix anything serious since the chances of a client
creating and destroying a mass of dumb BOs is pretty low.  However, it
is called by the other two create IOCTLs to garbage collect old objects.
Call it here too for consistency.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723172142.3273510-4-jason@jlekstrand.net