i965: Write gl_FragCoord directly to the destination.

i965: Write gl_FragCoord directly to the destination.

This patch makes emit_general_interpolation take a destination register
as an argument, and write directly to that.  This is simpler than the
old approach of ralloc'ing a register, writing to that temporary, and
then making the caller emit per-component MOVs to copy it to the actual
destination.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>