drm/xe/guc: Handle RCU_MODE as masked from definition

drm/xe/guc: Handle RCU_MODE as masked from definition

guc_mmio_regset_write() had a flags for the registers to be added to the
GuC's regset list. The only register actually using that was RCU_MODE,
but it was setting the flags to a bogus value. From
struct xe_guc_fwif.h,

	#define GUC_REGSET_MASKED               BIT(0)
	#define GUC_REGSET_MASKED_WITH_VALUE    BIT(2)
	#define GUC_REGSET_RESTORE_ONLY         BIT(3)

Cross checking with i915, the only flag to set in RCU_MODE is
GUC_REGSET_MASKED. That can be done automatically from the register, as
long as the definition is correct.

Add the XE_REG_OPTION_MASKED annotation to RCU_MODE and kill the "flags"
field in guc_mmio_regset_write(): guc_mmio_regset_write_one() can decide
that based on the register being passed.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20230429062332.354139-3-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>