[ARM] pxafb: cleanup of the color format manipulation code

1. introduce var_to_depth() to calculate the color depth including the
   transparency bit

2. the conversion from 'fb_var_screeninfo' to LCCR3 BPP bits can be re-
   used by overlays (in OVLxC1), thus an individual pxafb_var_to_bpp()
   has been separated out.

3. pxafb_setmode() should really set the color bitfields correctly at
   begining, introduce a pxafb_set_pixfmt() for this

4. allow user apps to specify color formats within fb_var_screeninfo,
   and checking of this in pxafb_check_var() has been simplified as
   below:

   a) pxafb_var_to_bpp() should pass - which means a basically correct
      bits_per_pixel and color depth setting
   b) the RGBT bitfields are then forced into supported values by
      pxafb_set_pixfmt()

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Eric Miao <ycmiao@ycmiao-hp520.(none)>