drm/i915/gem/i915_gem_ttm_pm: Provide a couple of missing descriptions for 'flags' and remove some superfluous ones

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c:156: warning: Function parameter or member 'flags' not described in 'i915_ttm_backup_region'
 drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c:156: warning: Excess function parameter 'allow_gpu' description in 'i915_ttm_backup_region'
 drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c:156: warning: Excess function parameter 'backup_pinned' description in 'i915_ttm_backup_region'
 drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c:223: warning: Function parameter or member 'flags' not described in 'i915_ttm_restore_region'
 drivers/gpu/drm/i915/gem/i915_gem_ttm_pm.c:223: warning: Excess function parameter 'allow_gpu' description in 'i915_ttm_restore_region'

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230331092607.700644-10-lee@kernel.org