drm/amd: Drop debugfs access to the DPCD

There is already access to the DPCD from userspace through
`CONFIG_DRM_DP_AUX_CHARDEV`, so it's unnecessary to reinvent the wheel
with a set of extra debugfs nodes specific to amdgpu.

The character device interface behaves more like you would expect in that
you can seek/read/write all from the same file.

Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>