Move gralloc_gbm_handle_t accesses into gralloc_gbm.cpp

Move gralloc_gbm_handle_t accesses into gralloc_gbm.cpp

The only access to the gralloc_gbm_handle_t in gralloc.cpp is to set the
stride of allocated buffers. Move this into gralloc_gbm.cpp to isolate
accesses to the struct in one place.

Signed-off-by: Rob Herring <robh@kernel.org>