drm/i915: Bail out of pipe config compute loop on LPT

drm/i915: Bail out of pipe config compute loop on LPT

LPT is pch, so might run into the fdi bandwidth constraint (especially
since it has only 2 lanes). But right now we just force pipe_bpp back
to 24, resulting in a nice loop (which we bail out with a loud
WARN_ON). Fix this.

Cc: Chris Wilson <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
References: https://bugs.freedesktop.org/show_bug.cgi?id=93477
Signed-off-by: Daniel Vetter <[email protected]>
Tested-by: Chris Wilson <[email protected]>
Signed-off-by: Maarten Lankhorst <[email protected]>
Cc: [email protected]
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit f58a1acc7e4a1f37d26124ce4c875c647fbcc61f)
Signed-off-by: Jani Nikula <[email protected]>