drm/i915/psr: Allow setting I915_PSR_DEBUG_SU_REGION_ET_DISABLE via debugfs

drm/i915/psr: Allow setting I915_PSR_DEBUG_SU_REGION_ET_DISABLE via debugfs

Currently setting I915_PSR_DEBUG_SU_REGION_ET_DISABLE (0x20) via psr_debug
debugfs interface is not allowed. This patch allows it.

v3:
  - ensure psr is disabled/enabled if enable_psr2_su_region_et changes
  - remove extra space
v2: ensure that fastset is performed when the bit changes

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240529093849.1016172-6-jouni.hogander@intel.com