i965/vec4: Use the actual channels used in pack_uniform_registers

i965/vec4: Use the actual channels used in pack_uniform_registers

Previously, pack_uniform_registers worked based on the size of the uniform
as given to us when we initially set up the uniforms.  However, we have to
walk through the uniforms and figure out liveness anyway, so we migh as
well record the number of channels used as we go.  This may also allow us
to pack things tighter in a few cases.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>