drm/amdkfd: Allow building KFD on ARM64 (v2)

ifdef x86_64 specific code.
Allow enabling CONFIG_HSA_AMD on ARM64.

v2: Fixed a compiler warning due to an unused variable

CC: Mark Nutter <Mark.Nutter@arm.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Mark Nutter <Mark.Nutter@arm.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>