drm/i915: Add i915.enable_sagv modparam

drm/i915: Add i915.enable_sagv modparam

Currently we have no sane way to forcibly disable SAGV, which
makes debugging things a PITA. Manually poking at the pcode
mailbox with it's various SAGV/QGV/PSF formats is no fun,
and likely to be clobbered by the driver anyway.

Let's add a modparam for this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230322181219.5511-1-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>