drm: Remove unnecessary condition in drm_output_render reuse

drm: Remove unnecessary condition in drm_output_render reuse

This condition inside drm_output_render() checks if we can reuse the
existing renderer buffer for the primary plane; this occurs in
mixed-mode composition where a client buffer promoted to a plane has
changed, but the primary plane is unchanged.

We accomplish this by checking if there is no damage on the
primary/renderer plane, and then if there is already a renderer buffer
active on the primary plane: in that case, we can reuse the buffer we
already have.

There was a further condition checking if the width and height were
identical. This was designed to prevent against issues on mode changes.
However, runtime mode changes are already quite broken, and a mode
change will also cause damage on the full plane. We can simply remove
this condition.

Signed-off-by: Daniel Stone <daniels@collabora.com>