mem-cache: Fix priority of multi compressor

mem-cache: Fix priority of multi compressor

The priority queue comparator orders such that false gives the
entry a higher priority. Therefore, if it is desired to make
the entry with lowest decompression latency have higher priority,
the comparison must be inverted.

Can be tested with:
  MultiCompressor(compressors=[
    PerfectCompressor(decompression_latency=1),
    PerfectCompressor(decompression_latency=2)])

Where it is expected that compressor0 (the one with decomp lat
of 1) is always chosen.

Change-Id: I44acbf5f51c6e47efdd2a16fba9596935cf2eb69
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28367
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>