drm/amd/display: Don't restrict bpc to 8 bpc

This will let us pass the kms_hdr.bpc_switch IGT
test.

The reason the bpc restriction was required is
historical. At one point in time we were not falling
back to a lower bpc when we didn't have enough
bandwidth for the maximum bpc reported by a display.
This meant that we couldn't enable some high refresh
modes unless we limitted the bpc.

Starting with this patch the issue is fixed:
commit cbd14ae7 ("drm/amd/display: Fix
incorrectly pruned modes with deep color")

This patch implemented a fallback mechanism if mode
validation failed at the max bpc. This means users
now automatically get all modes that can be supported
by at least 6 bpc. The driver will enable the mode
with the highest possible bpc that is supported by
the display.

v2:
 - explain why this is no longer needed (Michel)
 - refer to commit that fixed bpc fallback (Michel)

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Sebastian Wick <sebastian.wick@redhat.com>
Cc: Vitaly.Prosyak@amd.com
Cc: Joshua Ashton <joshua@froggi.es>
Cc: dri-devel@lists.freedesktop.org
Cc: amd-gfx@lists.freedesktop.org
Cc: Michel Dänzer <michel.daenzer@mailbox.org>
Reviewed-by: Joshua Ashton <joshua@froggi.es>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>