drm/i915: fix a read size argument

We want to read 3 bytes here, but because the parenthesis are in the
wrong place we instead read:

	sizeof(intel_dp->edp_dpcd) == sizeof(intel_dp->edp_dpcd)

which is one byte.

Fixes: fe5a66f9

 ("drm/i915: Read PSR caps/intermediate freqs/etc. only once on eDP")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <drm-intel-fixes@lists.freedesktop.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161013085508.GJ16198@mwanda