drm/i915/mtl: Don't set PIPE_CONTROL_FLUSH_L3

drm/i915/mtl: Don't set PIPE_CONTROL_FLUSH_L3

This bit does not cause an explicit L3 flush. We already use
PIPE_CONTROL_DC_FLUSH_ENABLE for that purpose.

v2: Use FLUSH_L3 only pre-MTL since spec will likely remain
the same going forward.

Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231017195309.2476088-1-vinay.belgaumkar@intel.com