drm/amdgpu: add rebar parameter

Add a new parameter to disable BAR resizing.  Note that this
only disables the driver from attempting to resize the BAR,
The BIOS may have resized the BAR at boot.

Some teams have found this useful in debugging P2P DMA
issues on systems where the available MMIO space did not allow
for all of the GPUs present to resize their BARs.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>