drm/gem-cma-helper: Change the level of the allocation failure message

drm/gem-cma-helper: Change the level of the allocation failure message

drm_gem_cma_create() prints an error message when dma_alloc_wc() fails to
allocate the amount of memory we requested. This can lead to annoying
error messages when CMA is only one possible source of memory for the BO
allocation. Turn this error message into a debug one.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20171005112917.15949-1-boris.brezillon@free-electrons.com