modesetting: Create a drmmode_bo wrapper; use it for front_bo.

This code is going to be extended to support GBM BOs soon.  This small
abstraction removes a lot of direct dumb_bo access, so we can add that
support in one place, rather than putting conditionals at every
pitch/handle/etc access.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Keith Packard <keithp@keithp.com>