drm/managed: Clean up GFP_ flag usage in drmm_kmalloc()

This code is not using the correct gfp flags which were passed in.
However, this does not affect runtime because kstrdup_const() is a
no-op in this context.  (It just returns the "kmalloc" string literal
without doing an allocation.)

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/ddf86b59-696a-45f0-96dd-b87aa7b9ab2e@moroto.mountain