drm/linux-core: drmP.h compilation fix

I need the following patch to fix compilation of
latest drm/linux-core on my ppc64 machine.

/home/mb/develop/git/drm/linux-core/savage_bci.c: In function ‘savage_driver_firstopen’:
/home/mb/develop/git/drm/linux-core/savage_bci.c:587: error: ‘DRM_MTRR_WC’ undeclared (first use in this function)
/home/mb/develop/git/drm/linux-core/savage_bci.c:587: error: (Each undeclared identifier is reported only once
/home/mb/develop/git/drm/linux-core/savage_bci.c:587: error: for each function it appears in.)
/home/mb/develop/git/drm/linux-core/savage_bci.c: In function ‘savage_driver_lastclose’:
/home/mb/develop/git/drm/linux-core/savage_bci.c:664: error: ‘DRM_MTRR_WC’ undeclared (first use in this function)

I looked at in-kernel drmP.h and it actually
has the same fix in it.

Signed-off-by: Michael Buesch <mb@bu3sch.de>