core: Provide own dumb graphics buffer allocator

If a gbm_bo is allocated with GBM_BO_USE_WRITE, it will be backed by a
dumb buffer under the hood. However, it seems like neither gbm_bo_get_fd()
nor gbm_bo_get_plane_fd() would return valid file descriptors, which are
required to fill in DmaBufAttributes.

As an interim solution, this change makes the GbmGraphicsBufferAllocator
allocate dumb buffers on its own rather than delegate it to gbm.