drm/modes: Print the mode status in human readable form

Currently when a mode is rejected the reason is printed as a raw number.
Having to manually decode that to a enum drm_mode_status value is
tiresome. Have the code do the decoding instead and print the result
in a human readable format.

Just having an array of strings indexed with the mode status doesn't
work since the enum includes negative values. So we offset the status
by +3 which makes all the indexes non-negative. Also add a bit of
paranoia into the code to catch out of bounds accesses in case
someone adds more enum values but forgets to update the code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>