drm/i915/vrr: Program EMP_AS_SDP_TL for DP AS SDP

The register EMP_AS_SDP_TL (MTL) was introduced for configuring the
double buffering point and transmission line for all
HDMI2.1 Extended Metadata Packets (VT-EMP for VRR, CVT-EMP for DSC etc).
This was also intended to be configured for DP to HDMI2.1 PCON to
support VRR.

From BMG and LNL+ onwards, this register was extended to Display Port
Adaptive Sync SDP to have a common register to configure double
buffering point and transmission line for both HDMI EMPs and DP VRR related
packets.

Currently, we do not support VRR for either native HDMI or via PCON.
However we need to configure this for DP SDP case. As per the spec,
program the register to set Vsync start as the double buffering point
for DP AS SDP.

v2:
-Make the helper more readable. (Jani)
-Add more information in commit message and comment.

Bspec:70984, 71197
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Tested-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://lore.kernel.org/r/20250505033911.393628-1-ankit.k.nautiyal@intel.com