drm/i915/perf: move pollin setup to non hw specific code

This isn't really gen specific stuff, so just move it to the common
code.

v2: Rebase (Umesh)
v3: Remove comment, pollin is a per stream state already (Ashutosh)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200324185457.14635-3-umesh.nerlige.ramappa@intel.com