i965/fs: remove special case in setup_payload_interference()

i965/fs: remove special case in setup_payload_interference()

regs_read() will handle LINTERP for us since the previous commit. In
addition, we were being too conservative, since it will only read 2
registers on SIMD8.

instructions in affected programs:     9061 -> 8893 (-1.85%)
helped:                                10
HURT:                                  0
GAINED:                                0
LOST:                                  0

All of the changes were due to spills being eliminated, mostly in KSP
shaders.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Connor Abbott <connor.w.abbott@intel.com>