[MIPS] Malta: set up the screen info in a separate function

This patch adds a separate short and sweet function to initialize
the screen_info global variable.

This improves readability of the Malta board setup code.

No functional changes introduced.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>