drm/i915/psr: Move logic to get TPS registers values to another function

This will make hsw_activate_psr1() more easy to read and will make
future modification to TPS registers more easy to review and read.

v4: Rename new function to intel_psr1_get_tp_time() (Dhinakaran and
Rodrigo)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312195743.8829-2-jose.souza@intel.com