[PATCH 1/5] fbcmap.c: mark structs const or __read_mostly

- Mark the default colormaps read-only, as nobody should be allowed to modify them
- Additionally mark color values as __read_mostly since they will only be modified (very seldom) by fb_invert_cmaps()
- Add named initializers in fb_cmap structs and use the ARRAY_SIZE() macro

Signed-off-by: Helge Deller <deller@gmx.de>