drm/amdgpu: Fix even more out of bound writes from debugfs

CVE-2021-42327 was fixed by:

commit f23750b5
Author: Thelford Williams <tdwilliamsiv@gmail.com>
Date:   Wed Oct 13 16:04:13 2021 -0400

    drm/amdgpu: fix out of bounds write

but amdgpu_dm_debugfs.c contains more of the same issue so fix the
remaining ones.

v2:
	* Add missing fix in dp_max_bpc_write (Harry Wentland)

Fixes: 918698d5 ("drm/amd/display: Return the number of bytes parsed than allocated")
Signed-off-by: Patrik Jakobsson <pjakobsson@suse.de>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org