davinci: fb: add framebuffer blank operation

Implement frame buffer blank operation feature for DA8xx/OMAP-L1xx driver.

Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>