video: fbdev: aty: Fix set but not used warnings in mach64_ct

Fix W=1 warnings about variables assigned but never used.
- One variable is only used when CONFIG_FB_ATY_GENERIC_LCD is defined
  Fix so variable is only defined with CONFIG_FB_ATY_GENERIC_LCD
- Several variables was only assigned by a call to aty_ld_le32().
  Drop the variables but keep the call to aty_ld_le32() as it may
  have unexpected side-effects.

v2:
  - Updated subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-7-sam@ravnborg.org