i965/fs: Fix broken math on values loaded from uniform buffers on gen6.

In a debug build this led to assertion failures, but on a non-debug
build the hardware would just reference the whole vec8 instead of the
same channel 8 times.

Fixes the new piglit glsl-1.40/uniform-buffer/fs-exp2.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57121
Note: This is a candidate for the stable branches
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 7b0731d940)

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -1929,6 +1929,7 @@ fs_visitor::register_coalesce()
 
       bool has_source_modifiers = (inst->src[0].abs ||
                                    inst->src[0].negate ||
+                                   inst->src[0].smear != -1 ||
                                    inst->src[0].file == UNIFORM);
 
       /* Found a move of a GRF to a GRF.  Let's see if we can coalesce