drm/amdgpu: Return boolean types instead of integer values

Return statements for functions returning bool should use truth
and false instead of 1 and 0 respectively.

Modify cik_event_interrupt.c to return false instead of 0.

Issue found with Coccinelle.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>