drm/i915: Detect eDRAM with the enabled bit only

At the moment we compare the whole EDRAM_PRESENT/EDRAMCAP register value
to 1 while EDRAM_PRESENT is only bit 0 (the rest may be used to describe
eDRAM capabilities).

To be more future proof, only look at bit 0 to detect eDRAM presence.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>