drm/i915/tgl: Read SAGV block time from PCODE

Starting from TGL, we now need to read the SAGV block time via a PCODE
mailbox, rather than having a static value.

BSpec: 49326

v2: Fix up pcode val data type (Ville), tighten variable scope (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: James Ausmus <james.ausmus@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004221449.1317-2-james.ausmus@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20191009172315.11004-2-lucas.demarchi@intel.com