drm/amd/display: Linux set/read lane settings through debugfs

 function: get current DP PHY settings: voltage swing, pre-emphasis,
 post-cursor2 (defined by VESA DP specification)

 valid values:  voltage swing: 0,1,2,3  pre-emphasis : 0,1,2,3
 post cursor2 : 0,1,2,3

 debugfs file phy_setings is located at  /sys/kernel/debug/dri/0/DP-x

 there will be directories, like DP-1, DP-2,DP-3, etc. for DP display

 --- to figure out which DP-x is the display for DP to be check,
 cd DP-x
 ls -ll
 There should be debugfs file, like link_settings, phy_settings.
 cat link_settings
 from lane_count, link_rate to figure which DP-x is for display to be
 worked on

 --- to get current DP PHY settings,
 cat phy_settings

 --- to change DP PHY settings,
 echo <voltage_swing> <pre-emphasis> <post_cursor2> > phy_settings

 for examle, to change voltage swing to 2, pre-emphasis to 3,
 post_cursor2 to 0,
 echo 2 3 0 > phy_settings

 ---  to check if change be applied, get current phy settings by
 cat phy_settings

 ---  in case invalid values are set by user, like
 echo 1 4 0 > phy_settings

 HW will NOT be programmed by these settings.

cat phy_settings will show the previous valid settings.
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>