Different DRM/KMS backends have a variable set of limitations, which is
not always exposed via DRM ioctls.

This implementation of backend-dependent validation provides a register
of platform-specific inherited backend class to the map.

The map key is a string that contains the corresponding DRM driver name.
During DrmHwcTwo class initialization the driver name will be checked and
a backend will be chosen. If the map does not have any backend for the
named driver, generic display backend will be used.

Signed-off-by: Matvii Zorin <matvii.zorin@globallogic.com>