relases gbm buffers on init
Created by: Ongy
The wlr_drm_surface_init function is called (upon others) when the drm mode is changed. When the surface was used previously this replaced the gbm_surface, but did not replace the gbm buffers (front/back). With this, wlr_drm_surface_get_from never set up the new buffers with the new glViewport because surf->front existed.
This frees the buffers to get new buffers on the new surface with the new viewport.
This fixes https://github.com/swaywm/wlroots/issues/593
Testplan: I should rebase the test-code mentioned in the issue, then use that branch with the keybind.