drm: msm: Fix build when legacy fbdev support isn't set

The DRM_KMS_FB_HELPER config is selected only when DRM_MSM_FBDEV config is
selected. The driver accesses drm_fb_helper_* functions even when legacy fbdev
support is disabled in msm. Wrap around these functions with #ifdef checks to
prevent build break.

Signed-off-by: Archit Taneja <architt@codeaurora.org>