drm/i915: prevent FIFO calculation overflows on 32 bits with high dotclocks

drm/i915: prevent FIFO calculation overflows on 32 bits with high dotclocks

commit d660467c3ff2a0b7413e1b7a51452b34ffb49e5f upstream.

A very high dotclock (e.g. 229500kHz as reported by Anton) can cause
the entries_required variable to overflow, potentially leading to a
FIFO watermark value that's too low to support the given mode.  Split
the division across the calculation to avoid this.

Reported-by: Anton Khirnov <wyskas@gmail.com>
Tested-by: Anton Khirnov <wyskas@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>