[MIPS] Remove redundant r4k_blast_icache() calls

r4k_flush_cache_all() and r4k_flush_cache_mm() case: these are noop if
the CPU did not have dc_aliases.  It would mean we do not need to care
about icache here.

r4k_flush_cache_range case: if r4k_flush_cache_mm() did not need to
care about icache, it would be same for this function.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>