meta: Fix reading luminance texture as rgba in _mesa_meta_pbo_GetTexSubImage()

meta: Fix reading luminance texture as rgba in _mesa_meta_pbo_GetTexSubImage()

After recent addition of pbo testing in piglit test getteximage-luminance,
it fails on i965. This patch makes a sub test pass.

This patch adds a clear color operation to meta pbo path, which I think is
better than falling back to software path.

V2: Fix color mask for GL_LUMINANCE_ALPHA

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>