drm/i915: Move SAGV block time to dev_priv

In prep for newer platforms having more complicated ways to determine
the SAGV block time, move the variable to dev_priv, and extract the
setting to an initial setup function. While we're at it, update the if
ladder to follow the new gen -> old gen order preference, and warn on
any non-specified gen.

v2: Shorten the function name (Ville), return directly (Ville), move
sagv_block_time_us value to dev_priv (Ville)

v3: Change sagv_block_time_us to u32 (Lucas), Change fallback value to
-1 (Lucas), use intel_has_sagv for setup check rather than hand-rolling
(Lucas)

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/20191004221449.1317-1-james.ausmus@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20191009172315.11004-1-lucas.demarchi@intel.com