drm/xe/oa: Preserve oa_ctrl unused bits

UMD's have interest in setting unused bits of the oa_ctrl register "out of
band" for certain experiments. To facilitate this, don't clobber previous
oa_ctrl unused bits, i.e. rmw the values rather than simply write them.

Fixes: e936f885 ("drm/xe/oa/uapi: Expose OA stream fd")
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117032155.3048063-1-ashutosh.dixit@intel.com