drm/i915: remove potentially confusing IS_G4X checks

The IS_G4X macro is defined as IS_G45 || IS_GM45. We have two points
in our code where we have an if statement checking for GM45 followed
by an else if statement checking for IS_G4X. This can be confusing
since the IS_G4X check won't be catching the previously-checked GM45.
Someone quickly trying to check which functions run on each platform
may end up getting confused while reading the code.

Fix the potential confusion by limiting the else if statements to only
check for the platform that was not already checked earlier in the if
ladder.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487620842-22893-3-git-send-email-paulo.r.zanoni@intel.com