backends/drm: Import gbm bos using dmabuf

It appears that importing gbm_bo's using eglCreateImageKHR() doesn't
work on nvidia.

Another issue is that Platform::createDmaBufTexture() uses
gbm_bo_create(). The nvidia driver doesn't like that, it's preferred to
use gbm_bo_create_with_modifiers().

In order to address those issues, this change refactors how gbm_bo
objects are imported and how gbm_bo's are allocated for dmabuf textures,
so the same code can be reused in EglGbmBackend::textureForOutput()
and Platform::createDmaBufTexture().