drm_hwcomposer: allow to force mode by a property (v5)

drm_hwcomposer: allow to force mode by a property (v5)

The desired resolution can be set by property debug.drm.mode.force.
The other modes are ignored.

(v2) Resolve conflict after commit 13cc366
     "drm_hwcomposer: use CamelCase in source/header files related to class"

(v3) Changes due to commit a148f21
     "drm_hwcomposer: Rework display modes handling"

(v4) Changes due to commit 650299a2
     "drm_hwcomposer: Tidy-up DrmConnector class"

(v5) Changes due to commit 7d89911c
     "drm_hwcomposer: Tidy-up DrmDevice class"