staging: sm750fb: add drm development branch details

The sm750 driver in staging has no future as new framebuffer driver
will not be added to fbdev anymore. A drm driver is being prepared, and
the details added to the TODO file. This sm750fb driver will be removed
as soon as the drm driver is ready. Also, remove my old email.

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>