msm: kgsl: Enhance reading lm_sequence in _execute_reg_sequence()

Modified conditional checks to read lm_sequence so as to avoid
the possibility of pointer overflow.

CRs-Fixed: 2212443
Change-Id: I72b30e35996c40f23fc81739e27724b1188f1c05
Signed-off-by: Archana Sriram <apsrir@codeaurora.org>