msm: kgsl: Restore the value of RB_BC_CONTROL on context switch

On 8x25Q, when GPU footswitch is turned off during TCXO shutdown, the
RB_BC_CONTROL register value is lost which causes GPU hang after coming
out of TCXO shutdown. Now the value of RB_BC_CONTROL is preserved properly
during context save and restore.

(cherry picked from commit 6b761ba560847bd7df4c6ff9de87bd6298d4b6e2)

Change-Id: I1999f88a5416445568c4b63ca0b99233551258f4
CRs-fixed: 441134
Signed-off-by: Vinay Roy <vroy@codeaurora.org>