drm: gma500: fix logic error

gcc-8 points out a condition that almost certainly doesn't
do what the author had in mind:

drivers/gpu/drm/gma500/mdfld_intel_display.c: In function 'mdfldWaitForPipeEnable':
drivers/gpu/drm/gma500/mdfld_intel_display.c:102:37: error: bitwise comparison always evaluates to false [-Werror=tautological-compare]

This changes it to a simple bit mask operation to check
whether the bit is set.

Fixes: 026abc33 ("gma500: initial medfield merge")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170905074741.435324-1-arnd@arndb.de