drm/vmwgfx: Simplify devcaps code

Make devcaps code self-contained so that it's easier to cache
and operate on them.
As the number of devcaps got bigger the code dealing with them
got more and more tricky. Lets create a central place to deal
with all the complexity. This lets us remove the lock we used
to require to deal with register write races because we only
read the devcaps at initialization.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609172307.131929-2-zackr@vmware.com