backend/drm: don't unset EGL context before swapping buffers
drm_fb_lock_surface swaps buffers. We need to make sure that the EGL context is current before calling wlr_egl_swap_buffers.
Move the wlr_egl_unset_current call into drm_fb_lock_surface to avoid other surprises like this.
Fixes: d28a7da9 ("backend/drm: add missing wlr_egl_unset_current") Closes: https://github.com/swaywm/wlroots/issues/2209
This is ready for review, but let's wait for user feedback before merging it, in case I missed something again.