drm/vmwgfx: Don't cache framebuffer maps

Buffer objects need to be either pinned or reserved while a map is active,
that's not the case here, so avoid caching the framebuffer map.
This will cause increasing mapping activity mainly when we don't do
page flipping.

This fixes occasional garbage filled screens when the framebuffer has been
evicted after the map.

Since in-kernel mapping of whole buffer objects is error-prone on 32-bit
architectures and also quite inefficient, we will revisit this later.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Cc: <stable@vger.kernel.org>