soc: qcom: ipa: kill struct ipa_dp

There's no need to enclose the "wrapper" slab cache in a structure,
nor to dynamically allocate that structure.  Just put the two cache
pointers directly in the IPA context structure.  Rename the functions
that set up/tear down those caches ipa_dp_init() and ipa_dp_exit().

Signed-off-by: Alex Elder <elder@linaro.org>