drm/tegra: Use new drm_fb_helper functions

Use the newly created wrapper drm_fb_helper functions instead of calling
core fbdev functions directly. They also simplify the fb_info creation.

v2:
- Fix up error handling path in tegra_fbdev_probe

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: "Terje Bergström" <tbergstrom@nvidia.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>