i965/fs: Fix stride field for uniforms.

i965/fs: Fix stride field for uniforms.

This fixes essentially the same problem as for immediates.  Registers
of the UNIFORM file are typically accessed according to the formula:

 read_uniform(r, channel_index, array_index) =
    read_element(r, channel_index * 0 + array_index * 1)

Which matches the general direct addressing formula for stride=0:

 read_direct(r, channel_index, array_index) =
    read_element(r, channel_index * stride +
                    array_index * max{1, stride * width})

In either case if reladdr is present the access will be according to
the composition of two register regions, the first one determining the
per-channel array_index used for the second, like:

 read_indirect(r, channel_index, array_index) =
    read_direct(r, channel_index,
                read(r.reladdr, channel_index, array_index))

where:
 read(r, channel_index, array_index) = if r.reladdr == NULL
    then read_direct(r, channel_index, array_index)
    else read_indirect(r, channel_index, array_index)

In conclusion we can handle uniforms consistently with the other
register files if we set stride to zero.  After lowering to a GRF
using VARYING_PULL_CONSTANT_LOAD in demote_pull_constant_loads() the
stride of the source is set to one again because the result of
VARYING_PULL_CONSTANT_LOAD is generally non-uniform.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>