drm/mgag200: fix a test in mga_vga_mode_valid()

The parentheses are in the wrong place here so we pass the bits per
pixel as zero.

Fixes: abbee623 ("drm/mgag200: Added resolution and bandwidth limits for various G200e products.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180125142655.GA23885@mwanda