drm/i915/gt: Rename flags with bit_group_X according to the datasheet

In preparation of the next patch align with the datasheet (BSPEC
47112) with the naming of the pipe control set of flag values.
The variable "flags" in gen12_emit_flush_rcs() is applied as a
set of flags called Bit Group 1.

Define also the Bit Group 0 as bit_group_0 where currently only
PIPE_CONTROL0_HDC_PIPELINE_FLUSH bit is set.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: <stable@vger.kernel.org> # v5.8+
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230725001950.1014671-5-andi.shyti@linux.intel.com
(cherry picked from commit f2dcd21d

)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>