drm/amdkfd: remove redundant variables err and ret

Return value from kfd_wait_on_events() and io_remap_pfn_range() directly
instead of taking this in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jinpeng Cui <cui.jinpeng2@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>