drm/radeon: Add local 'fb' variables

Add a local 'fb' variable to a few places to get rid of the
'crtc->primary->fb' stuff. Looks neater and helps me with my poor
coccinelle skills later.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-3-git-send-email-ville.syrjala@linux.intel.com