arm: tegra: display: provide framebuffer clear function

Add tegra_clear_framebuffer function.

bug 1175957

Change-Id: I12c249e011ecd839bbe9c5371b8be6e8a4b27bba
Signed-off-by: Jong Kim <jongk@nvidia.com>
Reviewed-on: http://git-master/r/170661
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Jon Mayo <jmayo@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Winnie Hsu <whsu@nvidia.com>

Conflicts:
	arch/arm/mach-tegra/board.h