drm/vmwgfx: Use the cpu blit utility for framebuffer to screen target blits

This blit was previously performed using two large vmaps, one of which
was teared down and remapped on each blit. Use the more resource-
conserving TTM cpu blit instead.

The blit is used in boundary-box computing mode which makes it possible
to minimize the bounding box used in host operations.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>