drm/i915/display: BMG supports UHBR13.5

UHBR20 is not supported by battlemage and the maximum link rate
supported is UHBR13.5

v2: Replace IS_DGFX with IS_BATTLEMAGE (Jani)

HSD: 16023263677
Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Fixes: 98b1c87a ("drm/i915/xe2hpd: Set maximum DP rate to UHBR13.5")
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240827081205.136569-1-arun.r.murthy@intel.com