drm/amd/display: increasing DRAM BW percent for DCN315

[why]
DML validation fails when we connect two or
more displays with HDR. Need to increase
DRAM BW to make the validation passing.
Following the value from DCN31.

[how]
Change the max DRAM BW DML field to 60%.

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Sung Joon Kim <sungkim@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>