glsl: fix count_attribute_slots to allow for different 64-bit handling

glsl: fix count_attribute_slots to allow for different 64-bit handling

So vertex shader input attributes are handled different than internal
varyings between shader stages, dvec3 and dvec4 only count as
one slot for vertex attributes, but for internal varyings, they
count as 2.

This patch comments all the uses of this API to clarify what we
pass in, except one which needs further investigation

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>