Add a resource manager class that is responsible for detecting all
kms devices and allocates unique display numbers for every detected
display.

This is controlled by the value of hwc.drm.device property, if it ends
with a %, it will try to open minor devices until an error is detected.
E.g: /dev/dri/card%

Additionally, this will be used for finding an available writeback
connector that will be used for the flattening of the currently
displayed scene.

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>