msm: kgsl: Don't read CP_RB_RPTR register without clock

msm: kgsl: Don't read CP_RB_RPTR register without clock

For A3xx we get GPU read pointer from the CP_RB_RPTR
register instead of rptr scratch memory address. In
retire_cmdbatch() and _retier_maraker() GPU clock will
be off, so avoid reading CP_RB_RPTR register. Also hold
device mutex in sendcmd() to access GPU registers.

CRs-Fixed: 1024730
Change-Id: Ifa5e9d3f892301685cb48a227ce4967d895499b1
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>