msm: kgsl: Change prev_padding type in subbuf_start_handler

The .subbuf_start callback expects the prev_padding type to be
size_t. Change the definition of subbuf_start_handler to use
size_t instead of uint.

Change-Id: I572ccf7adceb0c7f1fd24c01ff09abc111981016
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>