fbdev: imxfb: use BIT, FIELD_{GET,PREP} and GENMASK macros

Replace opencoded masking and shifting, with BIT(), GENMASK(),
FIELD_GET() and FIELD_PREP() macros.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Helge Deller <deller@gmx.de>