msm: kgsl: Correct gpu busy cycles calculations for LM on A640

When adjustment cycle counts are higher than gpu busy cycles,
prevent underflow by dropping adjustment cycles.

Change-Id: I4493d52d581d13ef23f34a64a66881cfd98e0c5c
Signed-off-by: Mohammed Mirza Mandayappurath Manzoor <mmandaya@codeaurora.org>