tegra: Fill in some details

Implement DRM buffer object handling and add support for clearing a
buffer. This allows a very basic OpenGL ES program, calling glClear(),
to run.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
tegra: format clean-up
tegra: round up height to multiples of tile-size
tegra: use linear staging-textures
tegra: only align 2d-resources
tegra: implement fences
tegra: perform blits with gr2d
tegra: perform transfers with blits
tegra: non-fullscreen clears
tegra: do not crash on NULL vertex-state object
tegra: no more user buffers
tegra: add comments for regsiter names in clear
st/clear: use clear_render_target and clear_depth_stencil for scissored clears
tegra: keep track of resource-layout
tegra: start grokking TGSI