drm/amdkfd: Correct SMI event read size

sizeof(buf) is 8 bytes because it is defined as unsigned char *buf,
each SMI event read only copy max 8 bytes to user buffer. Correct this
by using the buf allocate size.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>