drm/amd: correct "_LENTH" mispelling in constant

Correct the "_LENTH" mispelling in the AMDGPU_MAX_TIMEOUT_PARAM_LENGTH
constant.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>