drm/msm: Set memory retention flags on the GPU core clock

After enabling the GPU clocks, the GPU can pagefault
when trying to access memory(example the ringbuffer).
This patch addresses the pagefault issue by enabling
the memory retention flags on the GPU core clock.

Change-Id: Ibabecba77501d6a3b188b19c90c172de7d667c8c
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>