drm/komeda: Fix warning -Wunused-but-set-variable

Fixed two -Wunused-but-set-variable warnings:

/arm/linux/display/aosp-4.14-drm-next/drivers/gpu/drm/arm/display/komeda/komeda_kms.c: In function ‘komeda_crtc_normalize_zpos’:
/arm/linux/display/aosp-4.14-drm-next/drivers/gpu/drm/arm/display/komeda/komeda_kms.c:150:26: warning: variable ‘fb’ set but not used [-Wunused-but-set-variable]
  struct drm_framebuffer *fb;
                          ^~
/arm/linux/display/aosp-4.14-drm-next/drivers/gpu/drm/arm/display/komeda/komeda_kms.c: In function ‘komeda_kms_check’:
/arm/linux/display/aosp-4.14-drm-next/drivers/gpu/drm/arm/display/komeda/komeda_kms.c:209:25: warning: variable ‘old_crtc_st’ set but not used [-Wunused-but-set-variable]
  struct drm_crtc_state *old_crtc_st, *new_crtc_st;
                         ^~~~~~~~~~~

Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Reviewed-by: Ayan Kumar Halder <ayan.halder@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812112322.15990-1-james.qian.wang@arm.com