drm/radeon: Init amdkfd only if it was compiled

This patch changes the radeon_kfd_init(), which is used to initialize the
interface between radeon and amdkfd, so the interface will be initialized only
if amdkfd was build, either as module or inside the kernel image.

In the modules case, the symbol_request() will be used (same as old code). In
the in-image compilation case, a direct call to kgd2kfd_init() will be done.
For other cases, radeon_kfd_init() will just return false.

This patch is necessary because in case of the following specific
configuration: kernel 32-bit, no modules support, random kernel base and no
hibernation, the symbol_request() doesn't work as expected - it doesn't return
NULL if the symbol doesn't exists - which makes the kernel panic.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>