pan/midgard: Use type-appropriate swizzle for texture coordinate

pan/midgard: Use type-appropriate swizzle for texture coordinate

The texture coordinate for a 2D texture could be a vec2 or a vec3,
depending if it's an array texture or not. If it's vec2 (non-array
texture), we should not reference the z component; otherwise, liveness
analysis will get very confused when z is never written.

v2: Fix typo (Ilia).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>