drm: Consider drivers setting DRIVER_ATOMIC as atomic.

Drivers such as vc4 don't initialize mode_config.funcs until later in
initialization, but we know they're atomic since they've got the flag
set.  This avoids oopsing on dereferencing funcs in the new atomic
methods sanity checks.

I moved the atomic check function down below the core flag check, to
avoid needing a prototype.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: ba1f665f ("drm: Add checks for atomic_[duplicate/destroy]_state with atomic drivers")
Link: https://patchwork.freedesktop.org/patch/msgid/20180621195428.17447-1-eric@anholt.net


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>