backend-drm: Use resize_output to allow changing the fb

A video mode change would be needed to change the underlying renderer
framebuffer. All other backends make uses of this so let's do it for the
DRM-backend as well.

This would also be needed for the output capture to function properly as
we need call weston_output_update_capture_info() when a new mode set has
set. Otherwise we'd run into mismatched dimensions for the current mode
versus the dimensions set-up initially in weston_output_capture_source_info.

Signed-off-by: marius vlad <marius.vlad@collabora.com>