mesa: Add support for AMD_depth_clamp_separate

mesa: Add support for AMD_depth_clamp_separate

Enable _mesa_PushAttrib() and _mesa_PopAttrib() to handle
GL_DEPTH_CLAMP_NEAR_AMD and GL_DEPTH_CLAMP_FAR_AMD tokens.

Remove DepthClamp, because DepthClampNear + DepthClampFar replaces it,
as suggested by Marek Olsak.

Driver that enables AMD_depth_clamp_separate will only ever look at
DepthClampNear and DepthClampFar, as suggested by Ian Romanick.

v2: 1) Remove unnecessary parentheses (Marek Olsak)
    2) if AMD_depth_clamp_separate is unsupported, TEST_AND_UPDATE
       GL_DEPTH_CLAMP only (Marek Olsak)
    3) Clamp against near and far plane separately (Marek Olsak)
    4) Clip point separately for near and far Z clipping plane (Marek
       Olsak)

v3: Clamp raster position zw to the range [min(n,f), 0] for near plane
    and [0, max(n,f)] for far plane (Marek Olsak)

v4: Use MIN2 and MAX2 instead of CLAMP (Marek Olsak)

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>