drm/i915: Treat pre-gen4 backlight duty cycle value consistently

For i945 and earlier chips, the backlight frequency value had the low
bit (of 16) fixed to zero. The Pineview code path handled this by just
exposing the backlight range as 15 bits while other chips had the
backlight range limited to 0 .. 0xfffe.

This patch makes everyone take the pineview code path, providing 15
bits of backlight duty cycle range which seems more than sufficient to
me.

Daniel Mack reported that writing 1 to bit 0 of the duty cycle
register was causing problems on his Samsung X20 notebook, even when
the duty cycle value was less than the maximum backlight value. (He
tried a value of 29749 with max_brightness of 29750). This patch never
writes a '1' to that bit.

Signed-off-by: Keith Packard <[email protected]>
Reviewed-by: Takashi Iwai <[email protected]>
Reported-and-tested-by: Daniel Mack <[email protected]>
Cc: [email protected]

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