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>
glsl: Fix builtin_compiler build

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>