slim: msm: Disable runtime PM to avoid suspected slimbus lockup

It is suspected that runtime PM for slimbus controller may be
responsible for a slimbus register-access when slimbus core clock
is off.
Disable the runtime-PM for slimbus to avoid this scenario for now and
provide provision so that runtime-PM can be enabled from command line
if homescreen power needs to be reduced.

Change-Id: I6f9d8f00f5a5aefa56dfd8713172ed87c783e00d
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>