drm/msm/gem: move msm_gem cache flush api before copy

msm_gem object allocates the memory using kzalloc or vmalloc based on
memory size requirement. This is cacheable memory and must be flushed
before any cpu access on it.
This change moves the sync api before copying the data in the buffer to
make sure it has the recent content.

Change-Id: Ide39582023862b75dccdeb1461e1abd5f9977f94
Signed-off-by: Harsh Sahu <hsahu@codeaurora.org>