msm: mdss: stub bus scaling functions if driver is disabled

Add proper checks to disable bus scaling functions if the bus scaling
driver is not properly enabled. This allows driver to continue
operations even if bus driver is not enabled and bandwidth is voted
obtained in a different way.

CRs-Fixed: 987761
Change-Id: Ic5b0610ca2603b1f3a192ac5d777a512ad94f1b9
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>