meta: Add GLSL variant of _mesa_meta_GenerateMipmap() function

meta: Add GLSL variant of _mesa_meta_GenerateMipmap() function

This reduces the overhead of using the fixed function internally
in the driver.

V2: Use setup_glsl_generate_mipmap() and setup_ff_generate_mipmap()
    functions to avoid code duplication.
    Use glsl version when ARB_{vertex, fragmet}_shader are present.
    Remove redundant code.

V3: Remove redundant border related code leaving the assertion.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <idr@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>