drm/rockchip: Only alloc a kmap for fbdev gem object

In general, the data in drm/rockchip GEM objects is never accessed by
the kernel.  The objects are either accessed by a GPU, by display
controller DMA, or by mmap'ing them to user space.  Thus, these
buffers need not be mapped into kernel address space.

The only exception is the fbdev framebuffer(s), which may be written
in-kernel by fbcon.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>