drm/i915/psr: Make MASK_DISP_REG_WRITE reserved in PSR_MASK for ICL

ICL spec states that this bit is now reserved.

Bspec: 7722

v2(Dhinakaran and Jani):
- instead of remove bit in gen11 now only setting if if gen < 11
- changed commit title

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181003205031.32474-2-jose.souza@intel.com