drm/dp: add DPCD definitions from eDP 1.2

#define DP_EDP_DPCD_REV			    0x700    /* eDP 1.2 */

#define DP_EDP_GENERAL_CAP_1		    0x701

#define DP_EDP_BACKLIGHT_ADJUSTMENT_CAP     0x702

#define DP_EDP_GENERAL_CAP_2		    0x703

#define DP_EDP_DISPLAY_CONTROL_REGISTER     0x720

#define DP_EDP_BACKLIGHT_MODE_SET_REGISTER  0x721

#define DP_EDP_BACKLIGHT_BRIGHTNESS_MSB     0x722

#define DP_EDP_BACKLIGHT_BRIGHTNESS_LSB     0x723

#define DP_EDP_PWMGEN_BIT_COUNT             0x724

#define DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN     0x725

#define DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX     0x726

#define DP_EDP_BACKLIGHT_CONTROL_STATUS     0x727

#define DP_EDP_BACKLIGHT_FREQ_SET           0x728

#define DP_EDP_BACKLIGHT_FREQ_CAP_MIN_MSB   0x72a

#define DP_EDP_BACKLIGHT_FREQ_CAP_MIN_MID   0x72b

#define DP_EDP_BACKLIGHT_FREQ_CAP_MIN_LSB   0x72c

#define DP_EDP_BACKLIGHT_FREQ_CAP_MAX_MSB   0x72d

#define DP_EDP_BACKLIGHT_FREQ_CAP_MAX_MID   0x72e

#define DP_EDP_BACKLIGHT_FREQ_CAP_MAX_LSB   0x72f

#define DP_EDP_DBC_MINIMUM_BRIGHTNESS_SET   0x732

#define DP_EDP_DBC_MAXIMUM_BRIGHTNESS_SET   0x733

#define DP_SIDEBAND_MSG_DOWN_REQ_BASE	    0x1000   /* 1.2 MST */

#define DP_SIDEBAND_MSG_UP_REP_BASE	    0x1200   /* 1.2 MST */

#define DP_SIDEBAND_MSG_DOWN_REP_BASE	    0x1400   /* 1.2 MST */