drm/amdkfd: Fixed calculation of gart buffer size

This patch makes the gart's buffer size calculation more accurate. This buffer
is needed per GPU.

It takes into account maximum number of MQDs, runlist packets, kernel queues
and reserves 512KB for other misc allocations.

The total size is just shy of 4MB, for 32 processes and 128 queues per
process, which are the defaults for amdkfd kernel module parameters.

Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Alexey Skidanov <Alexey.skidanov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>