drm/i915: s/i9xx_/gm45_ for the gm45 write_infoframe function

Generally we call stuff with i9xx_ when it's valid for gen3+. But
gen3 and early gen4 only support hdmi with sdvo cards, and writing
infoframes works completely different there.

v2: Use g4x instead of gm45 - it applies to the desktop variant, too.

v3: Properly align the paramters of g4x_write_infoframe again, noticed
by Paulo Zanoni.

Acked-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>