drm/amd/display: Return the number of bytes parsed than allocated

[why & how]
Previously we were returning the number of bytes allocated
for a write buffer from debugfs and when manually used it wouldn't
rise any errors, but it wouldn't match the size of the parameters
passed from userspace.

In successful case return the size passed by usermode otherwise
the error code is returned. That simplifies the parser helper
and removes a potential error of returning mismatched input size.

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>