Implement drivers for NVIDIA tegra124 display controller, HDMI source

Implement drivers for NVIDIA tegra124 display controller, HDMI source
and host1x module. Unfortunately, tegra124 SoC doesn't have 2D acceleration
engine and 3D requires not yet started nouveau driver.

These drivers forms a first non-x86 DRM2 enabled graphic stack.

Note, there are 2 outstanding issues:
 - The code uses gross hack in order to be comply with
   OBJT_MGTDEVICE pager. (See tegra_bo_init_pager() in tegra_bo.c)
 - Due to improper(probably) refcounting in drm_gem_mmap_single()
   (in drm_gem.c), the gem objects are never released.
I hope that I will be able to address both issues in finite time,
but I don't want to touch x86 world now.

MFC after: 1 month