msm: kgsl: make the snapshot buffer a byte buffer

Add type safety to the snapshot section functions by making
the buffer be a 'u8 *' instead of a 'void *'. This makes
it easier to ensure that you don't accidently pass a pointer
to something else, like a struct kgsl_snapshot, into the wrong
function. Also rename the buffer argument to 'buf', to
avoid ambiguity with struct kgsl_snapshot arguments that
are called 'snapshot'.

Change-Id: I98e0a20062859a855aeb22d65aba1ceccee3e8af
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>