drm: extract legacy framebuffer remove

drm: extract legacy framebuffer remove

I got confused every time I audited what that lock_all is doing in
there until realizing it's for legacy kms only. Make that a notch more
obvious by having 2 entirely different paths.

While at it also move the atomic version of this into
drm_framebuffer.c, there's no reason it needs to be in drm_atomic.c.
That way it becomes a simple static function.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-15-daniel.vetter@ffwll.ch