compositor-drm: Renaming of gbm fields

In preparation for follow-on changes to support frame presentation
through EGLDevice+EGLOutput, this change includes the following:
  - Rename drm_backend::format   to gbm_format
  - Rename drm_output::format    to gbm_format
  - Rename drm_output::surface   to gbm_surface
  - Rename drm_output::cursor_bo to gbm_cursor_bo

Signed-off-by: Miguel A Vico Moya <mvicomoya@nvidia.com>
Reviewed-by: Andy Ritger <aritger@nvidia.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
[Pekka: trivial rebase out of the series]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>