drm/arm: 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 ppor
coccinelle skills later.

In some places the local variable was already there, just not used
consistently.

Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Brian Starkey <brian.starkey@arm.com>
Cc: Mali DP Maintainers <malidp@foss.arm.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-10-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>