drm/xe: Label RING_CONTEXT_CONTROL as masked

RING_CONTEXT_CONTROL is a masked register.

v2: Also clean up setting register value (Lucas)

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404161256.3852502-1-ashutosh.dixit@intel.com