drm/vmwgfx: Fix gem refcounting and memory evictions

v2: Add the last part of the ref count fix which was spotted by
Philipp Sieweck where the ref count of cpu writers is off due to
ERESTARTSYS or EBUSY during bo waits.

The initial GEM port broke refcounting on shareable (prime) surfaces and
memory evictions. The prime surfaces broke because the parent surfaces
weren't increasing the ref count on GEM surfaces, which meant that
the memory backing textures could have been deleted while the texture
was still accessible. The evictions broke due to a typo, the code was
supposed to exit if the passed buffers were not vmw_buffer_object
not if they were. They're tied because the evictions depend on having
memory to actually evict.

This fixes crashes with XA state tracker which is used for xrender
acceleration on xf86-video-vmware, apps/tests which use a lot of
memory (a good test being the piglit's streaming-texture-leak) and
desktops.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Fixes: 8afa13a0 ("drm/vmwgfx: Implement DRIVER_GEM")
Reported-by: Philipp Sieweck <psi@informatik.uni-kiel.de>
Cc: <stable@vger.kernel.org> # v5.17+
Reviewed-by: Maaz Mombasawala <mombasawalam@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220420040328.1007409-1-zack@kde.org