drm/i915/buddy: adjust res->start

Differentiate between mappable vs non-mappable resources, also if this
is an actual range allocation ensure we set res->start as the starting
pfn. Later when we need to do non-mappable -> mappable moves then we
want TTM to see that the current placement is not compatible, which
should result in an actual move, instead of being turned into a noop.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220225145502.331818-5-matthew.auld@intel.com