drm/radeon: make sure mode init is complete in bandwidth_update

The power management code calls into the display code for
certain things.  If certain power management sysfs attributes
are called before the driver has finished initializing all of
the hardware we can run into problems with uninitialized
modesetting state.  Add a check to make sure modesetting
init has completed to the bandwidth update callbacks to
fix this.  Can be triggered by the tlp and laptop start
up scripts depending on the timing.

bugs:
https://bugzilla.kernel.org/show_bug.cgi?id=83611
https://bugs.freedesktop.org/show_bug.cgi?id=85771



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org