i965: Separate gl_InstanceID and gl_VertexID uploading.

i965: Separate gl_InstanceID and gl_VertexID uploading.

We always uploaded them together, mostly out of laziness - both required
an additional vertex element.  However, gl_VertexID now also requires an
additional vertex buffer for storing gl_BaseVertex; for non-indirect
draws this also means uploading (a small amount of) data.  This is extra
overhead we don't need if the shader only uses gl_InstanceID.

In particular, our clear shaders currently use gl_InstanceID for doing
layered clears, but don't need gl_VertexID.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "10.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Ian Romanick <ian.d.romanick@intel.com>