Split enum gprs_pmm_state into Iu and Gb counterparts

Those two state sets are not part of the same state machine, and are
used in different scenarios, so let's split them and handle them in Gb
and Iu specific parts of struct sgsn_mm_ctx. This is required in order
to improve related code (for instance, use osmocom fsm).

Change-Id: I6100d607da316da0595886c6968704dd9ccfbde9