mm, vmpressure: int cast vmpressure level/model for -1 comparison

Resolve -Wenum-compare issue when comparing vmpressure level/model
against -1 (invalid state).

Change-Id: I1c76667ee8390e2d396c96e5ed73f30d0700ffa8
Signed-off-by: David Ng <dave@codeaurora.org>