gpu: nvgpu: Remove extraneous FB flush calls

gpu: nvgpu: Remove extraneous FB flush calls

gk20a_mm_fb_flush() invoked G_ELPG_FLUSH and FB_FLUSH. Remove the
invokation of G_ELPG_FLUSH. Replace calls to gk20a_mm_fb_flush() with
gk20a_mm_l2_flush() when appropriate.

Bug 1421824

Change-Id: I02af4bdc3b7bd26d0f6a8d610f70349269775a36
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/408210
(cherry picked from commit f02f34a8d214d883c949ab55fe872d4176a21bc5)
Reviewed-on: http://git-master/r/423233
Reviewed-by: Simone Willett <swillett@nvidia.com>
Tested-by: Simone Willett <swillett@nvidia.com>