fbdev: i810: include i810_main.h in i810_dvt.c

Building with W=1 shows that a header needs to be included to
make the prototypes visible:

drivers/video/fbdev/i810/i810_dvt.c:194:6: error: no previous prototype for 'round_off_xres' [-Werror=missing-prototypes]
drivers/video/fbdev/i810/i810_dvt.c:233:6: error: no previous prototype for 'i810fb_encode_registers' [-Werror=missing-prototypes]
drivers/video/fbdev/i810/i810_dvt.c:245:6: error: no previous prototype for 'i810fb_fill_var_timings' [-Werror=missing-prototypes]
drivers/video/fbdev/i810/i810_dvt.c:279:5: error: no previous prototype for 'i810_get_watermark' [-Werror=missing-prototypes]

Adding the header leads to another warning from a mismatched
prototype, so fix this as well:

drivers/video/fbdev/i810/i810_dvt.c:280:5: error: conflicting types for 'i810_get_watermark'; have 'u32(struct fb_var_screeninfo *,

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Helge Deller <deller@gmx.de>