drm/amd/display: fix crash/reboot while accessing sysfs files

read/writes to aux_dpcd_* sysfs entries leads to system
reboot or hang.
Hence fix the handling of input data and reporting of errors
appropriately to the user space.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>