drm/vkms: Fix typo and preposion in function documentation

Fix typo in word 'blend' and in the word 'destination' and change
preposition 'at' to 'of' in function 'blend' documentation.
And change the task introduction word 'Todo' for the word all in uppercase
- 'TODO'. With the TODO word all in uppercase (as it's the standard) it's
easier to find the tasks that have to be done throughout the code.

Changes since V3:
 Rodrigo:
 - Merge the patch series into a single patch since it contains one single
 logical change

Changes since V2:
 - Add fix typo in  word 'destination'
 - Add change of the preposition
 - Fix the name of the function in log message
 - Add the change in word 'Todo'

Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Gabriela Bittencourt <gabrielabittencourt00@gmail.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191104161424.18105-1-gabrielabittencourt00@gmail.com