msm: mdss: fix bus calculations overflows

With higher display resolutions the bus calculations being performed
can be very high which may cause overflow and unwanted results. To avoid
such situations modify logic to perform calculations in kb and use 64bit
data type to call into bus scaling driver.

Change-Id: I7b975d750e316c6e79bcdcd54a8194c8ae40c155
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>