drm/i915: Return a default RCS context size

Instead of returning whatever size the latest GEN used. This is because
context sizes for new GENs can go up or down, but the only safe thing to
do for missing cases is to use the largest known one, whatever that is.

Suggested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1515711307-28979-1-git-send-email-oscar.mateo@intel.com