drm: Fix misleading documentation of drm_gem_cma_create()

The backing memory does not necessarily have the writecombine attribute
set, since commit cf8ccbc7 ("drm: Add support for GEM buffers backed
by non-coherent memory").

Fixes: cf8ccbc7 ("drm: Add support for GEM buffers backed by non-coherent memory")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210601095405.8975-1-paul@crapouillou.net