drm/i915: Change N divider minimum from 3 to 2 for gen2

Bruno Prémont has a 855 machine with a 1400x1050 LVDS screen.

The VBT mode is as follows:
0:"1400x1050" 0 108000 1400 1416 1528 1688 1050 1051 1054 1066 0x8 0xa

The BIOS uses the following DPLL settings:
DPLL = 0x90020000
FP0 = 0x2140e
FP1 = 0x21207

We can't generate that pixel clock currently as we're limiting the N
divider to at least 3, whereas the BIOS uses a value of 2.

Let's reduce the N minimum to 2 and see what happens.

Cc: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>