drm/gpuva_mgr: generalize dma_resv/extobj handling and GEM validation

So far the DRM GPUVA manager offers common infrastructure to track GPU VA
allocations and mappings, generically connect GPU VA mappings to their
backing buffers and perform more complex mapping operations on the GPU VA
space.

However, there are more design patterns commonly used by drivers, which
can potentially be generalized in order to make the DRM GPUVA manager
represent a basic GPU-VM implementation. In this context, this patch aims
at generalizing the following elements.

1) Provide a common dma-resv for GEM objects not being used outside of
   this GPU-VM.

2) Provide tracking of external GEM objects (GEM objects which are
   shared with other GPU-VMs).

3) Provide functions to efficiently lock all GEM objects dma-resv the
   GPU-VM contains mappings of.

4) Provide tracking of evicted GEM objects the GPU-VM contains mappings
   of, such that validation of evicted GEM objects is accelerated.

5) Provide some convinience functions for common patterns.

Rather than being designed as a "framework", the target is to make all
features appear as a collection of optional helper functions, such that
drivers are free to make use of the DRM GPUVA managers basic
functionality and opt-in for other features without setting any feature
flags, just by making use of the corresponding functions.

Signed-off-by: Danilo Krummrich <dakr@redhat.com>