drm/i915: Add function to check for linear surfaces

A framebuffer can comprise surfaces with distinct tiling formats,
making checks against modifier alone insufficient. Make use of a
function to identify a linear surface based on both modifier and color
plane.

v2: Typo fix
v3: remove 'inline' from function definition (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181026193805.11077-1-dhinakaran.pandiyan@intel.com