staging: sm750fb: change default screen resolution

Update the default screen resolution and also use 24bpp for a better
screen performance.

Tested-by: Teddy Wang <teddy.wang@siliconmotion.com>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>