panfrost/midgard: Set masks on ld_vary

panfrost/midgard: Set masks on ld_vary

These masks distinguish scalar/vec2/vec3 loads from the default vec4,
which helps with assembly readability (since it's immediately obvious
how many components are _actually_ affected, rather than doing
mysterious things to an unknown number of unused components). Later in
the series, this will enable smarter register allocation, as the unused
components will not be interpreted abnormally.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Ryan Houdek <Sonicadvance1@gmail.com>