amdgpu: fix "add AMDGPU_VA_RANGE_HIGH"

The range is stored as exclusive, not inclusive. Subtracts one to get
the inclusive interval for the calculation. This fixes crashes when 32bit
addresses are in use.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>