backends/drm: Reopen the drm node

GEM handles are not reference counted and are unique per drm file
descriptor.

With the upcoming changes, dmabuf and dumb buffers will be abstraced
using DmaBufAttributes.

The issue with it is that we need GEM handles to get a drm framebuffer.
drmPrimeFDToHandle() can be used to get the GEM handle for a drm file
descriptor, but we would hit the reference counting issues then and
potentially double close GEM handles.

In order to resolve that, this change makes the drm backend reopen the
drm node to get a new GEM handle namespace for rendering.