drm/i915: Fix kzalloc() smatch warnings in get_initial_plane_config()

drm/i915: Fix kzalloc() smatch warnings in get_initial_plane_config()

Smatch doesn't like:

  struct drm_framebuffer *fb;
  fb = kzalloc(sizeof(struct intel_framebuffer), GFP_KERNEL);

and warns with:

  warn: struct type mismatch 'drm_framebuffer vs intel_framebuffer'

This implicit cast was correct as struct intel_framebuffer has struct
drm_framebuffer as its first member, but in case someone want to reorder
the fields for some reason, it's slightly safer to access the underlying
drm_framebuffer through intel_fb->base.

Also, having fewer static analysis warnings is a worthy goal.

Cc: kbuild@01.org
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>