mesa/core: Enable EXT_depth_clamp for GLES >= 2.0

mesa/core: Enable EXT_depth_clamp for GLES >= 2.0

The extension NV_depth_clamp is written against OpenGL 1.2.1, and
since GLES 2.0 is based on GL 2.0 there is no reason not to enable
this extension also for GLES >= 2.0.

v2: Use EXT_depth_clamp that has been proposed to Khronos

v3: - Fix check for extension availability (Erik Faya-Lund)
    - Also fix the test in is_enabled
v4: - Test both, ARB and EXT extension (Erik)
v5: - Fix white space errors (Erik)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>