drm/amd/display: Force link_rate as LINK_RATE_RBR2 for 2018 15" Apple Retina panels

The eDP link rate reported by the DP_MAX_LINK_RATE dpcd register (0xa) is
contradictory to the highest rate supported reported by
EDID (0xc = LINK_RATE_RBR2). The effects of this compounded with commit
'4a8ca46b ("drm/amd/display: Default max bpc to 16 for eDP")' results
in no display modes being found and a dark panel.

For now, simply force the maximum supported link rate for the eDP attached
2018 15" Apple Retina panels.

Additionally, we must also check the firmware revision since the device ID
reported by the DPCD is identical to that of the more capable 16,1,
incorrectly quirking it. We also use said firmware check to quirk the
refreshed 15,1 models with Vega graphics as they use a slightly newer
firmware version.

Tested-by: Aun-Ali Zaidi <admin@kodeit.net>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Aun-Ali Zaidi <admin@kodeit.net>
Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>