drm/vmwgfx: Be more restrictive when dirtying resources

Currently we flag resources as dirty (GPU contents not yet read back to
the backing MOB) whenever they have been part of a command stream.
Obviously many resources can't be dirty and others can only be dirty when
written to by the GPU. That is when they are either bound to the context as
render-targets, depth-stencil, copy / clear destinations and
stream-output targets, or similarly when there are corresponding views into
them.
So mark resources dirty only in these special cases. Context- and cotable
resources are always marked dirty when referenced.
This is important for upcoming emulated coherent memory, since we can avoid
issuing automatic readbacks to non-dirty resources when the CPU tries to
access part of the backing MOB.

Testing: Unigine Heaven with max GPU memory set to 256MB resulting in
heavy resource thrashing.
---
v2: Addressed review comments by Deepak Rawat.
v3: Added some documentation

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>