drm/i915/dsb: Add i915.enable_dsb module parameter

drm/i915/dsb: Add i915.enable_dsb module parameter

As we extend the use of DSB for critical pipe/plane register
programming, it'll be nice to have an escape valve at hand,
in case things go very poorly. To that end, add a i915.enable_dsb
modparam by which we can force the driver to take the pure mmio
path instead.

v2: Use 0400 permissions for the actual modparam (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611133344.30673-9-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>