drm/i915/frontbuffer: Pull frontbuffer_flush out of gem_obj_pin_to_display

i915_gem_obj_pin_to_display() calls frontbuffer_flush with origin set to
DIRTYFB. The callers however are at a vantage point to decide if hardware
frontbuffer tracking can do the flush for us. For example, legacy cursor
updates, like flips, write to MMIO registers, which then triggers PSR flush
by the hardware. Moving frontbuffer_flush out will enable us to skip a
software initiated flush by setting origin to FLIP. Thanks to Chris for the
idea.

v2:
Rebased due to Ville adding intel_plane_pin_fb().
Minor code reordering as fb_obj_flush doesn't need struct_mutex (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180307033420.3086-1-dhinakaran.pandiyan@intel.com