drm/amdkfd: Fix gfx8 MEM_VIOL exception handler

When MEM_VIOL is asserted the context save handler rewinds the
program counter. This is incorrect for any source of the exception.
MEM_VIOL may be raised in normal operation by out-of-bounds access
to LDS or GDS and does not require special handling.

Remove PC adjustment when MEM_VIOL has been raised.

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